There was an issue in earlier 386 code versions where DHCP requests from the guest wireless 1 (aimesh guest) VLANs were being forwarded upstream to the ISP out the WAN port. It was resolved (not sure what code but definitely in 386.7_2 on my AC1900), but before it was resolved it would cause internet to go down with most fiber carriers as their ONT was stripping the VLAN tag off, giving the guest device the IP, and the router would end up in a routing loop.
Are you running the latest code on both devices? Perhaps one of those models did not get the "fix". If you SSH in with Guest 1 enabled you can look and see if you have VLAN 501 and 502 configured on the AX88U's WAN port, in which case this could be your issue, it does sound pretty familiar to what a bunch of us experienced back then.
I had one guest device at the time and it renewed its IP every 24 hours and every 24 hours my internet went down. Finally got it figured out and disabled guest 1 (the one that uses VLANs and separate subnets), then eventually permanently fixed it by removing those VLANs from the WAN port. But found once I upgraded to 386.7_2 it was no longer necessary to remove them from the WAN port as they had fixed the problem.
Does it work in Aimesh without guest enabled? I guess that would be one step back toward "normal" you could try.
Thank you for all of this information.
I am running the latest code on both devices, yes, you can see I already mentioned it on the first post.
AiMesh router
RT-AX88U
Current Version : 3.0.0.4.388_22525-gd35b8fe
AiMesh Node
RT-AX56U
Current Version : 3.0.0.4.386_51665-g8072e52
You are saying you had your issue every 24hours, but in my case it's random, it could be 5 minutes and it could be 4 days, but it is possible it's the renew of IPs of Guest Network.
But your problem does not mention the WiFi connectivity issue that comes with the WAN down issue,
Once there is the random modem hangup issue there's also the issue with the WiFI of the Asus RT-AX88U, which I cannot connect, only to the Asus RT-AX56U WiFI I can connect.
So it seems your problem is not the same as mine?
I'm sorry, can you please explain more about the SSH part? I don't have much knowledge about that part, but I do know you need PuTTy in order to SSH, use IP 192.168.50.1 (Asus RT-AX88U),
username and password same as GUI, but then what commands do I use in order to test what you said?
Also - Should I test it before there is a problem or during the issue?
I am now testing the following:
Asus RT-AX88U (AiMesh Router) -> Asus RT-AX56U (AiMesh Node)
Settings: Guest network on AiMesh = Router only (Asus RT-AX56U does not broadcast Guest Network, only the Asus RT-AX88U)
I will update here after a week or once I encounter problems.