This might be a combination of EMI and device age as these things did not happened when the device was new, the adapter and sd card are the same as before.
I'm using some generic micro sdcard adapter, tested a different Ugreen adapter and also a Kingston 64GB DataTraveler pen (brand new), all with same results, 2.4 gHz WiFi disappears and reappears at random even if I'm near the router (2m), USB 2.0 mode fixed this, may also try the shielded cable if I can find a decent one.
Using USB 2.0 should not be a major issue as the pen is for Entware and related stuff, for high volume data I have a NAS.
I can't drop 2.4 gHz as many IoT devices I use only support that unfortunately.
As for temps should not be an issue as I have two 140mm Noctua fans blowing in the back of the router, similar setup I had for my previous RT-AC68U because of 90c CPU temps, this is overkill for this RT-AX86U though.
@Tech9 Range was not an issue here as even at close range there was no WiFi.
I now completed the setup and set again the wifi network name so all devices would connect and had a "Jun 16 02:26:37 RT-AX86U-2F78 kernel: wl0: fatal error, reinitializing, total count of reinit's[1]", maybe it got overwhelmed and crashed.
This also happened a few months ago but some reboots and factory resets "fixed" it, I was now getting mad as the warranty is about to expire and my previous devices still works fine in a friends house.
This device may eventualy transform into an AP as I'm planning into getting one of those N100 2.5G mini PC's and using ONPsense or simmilar (might be overkill for what I need though).
Thanks for the tips.
Kado