Asus RT-AX88U internet speed slows down after a few days

[scandalous]

I use scMerlin for some specific things, and not very often at that, but yes it works, see below...

"if I understand you correctly, there's no need if I use the scMerlin addon correct? The same settings will be exposed that I need to turn off as highlighted in your first screenshot?"
Yes!, at least it did for me back when I dealt with this early on in the 388.x life cycle. I don't remember if AiProtection had an impact as I never really enabled for any significant length of time.

I have accepted the EULA and Not Withdrawn Consent as I use their DDNS service, for what it's worth.

In in going back an forth between firmwares I did WPS resets which is why it drove me nuts (because I did the nodes as well, and had to rebuild AiMesh every single time). I did remember messing with those paramters at some point before I started having problemd, but I did have QoS disabled, so I thought no harm no foul when I stumbled upon it I unchecked everything as shown and set all the other parameters to 100%. But I do remeber quite vividly the moment I unchecked ACK, SYN and FIN and hit apply and saw my bandwidth (as measured via SpeedTest on the router) return to my normal and stay there.

As for it running on an AX88u on 388.9, here you go, and see my signature...
View attachment 65317

So I'm back to report that unfortunately your recommendations did not work. The speed halved a couple hours later. I looked through the logs as well, but I don't see anything out of the ordinary. Just some handshakes etc.

Standard reminder. If you are having speed issues. If you have enabled any of the AiProtection, TrendMicro features or any feature that utilizes either, you should disable those features (all of them) then withdraw from them on the Administration > Privacy (or Policy) page. Just disabling them isn't enough, you have to also withdraw permission for them on the Privacy/Policy page as well. Otherwise they may still run in the background. Then test to see if the issue continues.

When all else fails, perform a hard factory reset then a basic manual configuration (do not import a saved router.cfg file or attach a USB hard drive to the router). Don't enable extra features beyond the basics to get the router up and running. Don't add any addon scripts. Test the router as bear as possible after a reset to troubleshoot.

This also made no difference, the issue still occurs.

Just for giggles, if you have a CAT 6 cable handy, try that between the ONT and the Asus router. CAT7 is more sensitive to imperfections in the termination and receptacle.

If you want to run speed tests, they should be run from lan client device and not the router. Generally, not reasonably accurate when run on the router due to the CPU load.

I tried the cat6 cable and that made no difference either.

Well, you certainly getting a lot of advice... some sound and some just noise. In reality, your router should be able to run at the default settings just fine. Even with the Trend Micro features enabled! Messing with the WIFI settings can cause more issues.
As for the PPPoE MTU I mentioned before, the setting is generally recommended to be 1492 bytes. Some feel that this will slow the bandwidth when in fact it can improve it as fragmentation will be reduced. It may be worth your while to try it.
Yes, there can be other factors. Custom firmware included (I do not want to knock the Merlin effort and I have run his firmware off and on for years with good results). Going back to the factory firmware and running simple configuration should be a first start.

I appreciate all advice coming my way. I'll be trying the MTU set to 1492 next. If that doesn't work, I'll go back to the stock firmware and report back as soon as I can.

 

[scandalous]

Well, you certainly getting a lot of advice... some sound and some just noise. In reality, your router should be able to run at the default settings just fine. Even with the Trend Micro features enabled! Messing with the WIFI settings can cause more issues.
As for the PPPoE MTU I mentioned before, the setting is generally recommended to be 1492 bytes. Some feel that this will slow the bandwidth when in fact it can improve it as fragmentation will be reduced. It may be worth your while to try it.
Yes, there can be other factors. Custom firmware included (I do not want to knock the Merlin effort and I have run his firmware off and on for years with good results). Going back to the factory firmware and running simple configuration should be a first start.

Just wanted to confirm that the MTU default setting did not solve the problem. The issue is here again after I changed the setting and rebooted.

 

[degrub]

Does that router have an alternate LAN port that can be used for WAN ?

 

[scandalous]

Does that router have an alternate LAN port that can be used for WAN ?

Unfortunately it does not. I've consider a broken WAN port, but would find it really weird that it could work for up to 48 hours sometimes and then totally drop. If the issue persists on the stock Asus firmware, I'll consider it a hardware defect and get a new device.

 

[FernandoF]

@scandalous, if you want to try an alternate LAN port that can be used for WAN, there's a trick you can take advantage of:

1. Go to WAN > Dual WAN
2. Set "Enable Dual WAN" to ON
3. Choose a "Primary WAN" port that's different than the regular WAN port. The recommendation is to use LAN 1-4, as LAN 5-8 seem to be problematic with the RT-AX88U
4. You can probably set the "Secondary WAN" with the regular WAN port - it won't be used anyway
5. Leave the other fields untouched and Apply - the router will reboot
6. During the reboot, switch your WAN ethernet cable from the regular WAN port to the LAN port you selected in #3
7. After the reboot, go to WAN > Internet Connection, select the "WAN Type" to match the same LAN port from #3 and #6, adjust the other WAN fields properly (as little as possible), and Apply
8. Reboot

In case you do have a broken WAN port, the above may help. Good luck!