Thank you
@Boogieman for your effort to make media bridge stable.
PS: I'm new to the forum and it's possible the discoveries from the following story is already well known by others, or maybe is not recommended:
Asus RT-AC68U as main router AC/mixed 5GHz mode (Firmware: Asuswrt-Merlin 386.14_2)
Asus RT-AC66U as Media Bridge (Firmware: Asuswrt-Merlin 380.66_4)
(+some old N router with dd-wrt for IoT Wi-Fi in 2.4 Ghz B/G mode)
I initially had setup the RT-AC68U also with standard 2.4Ghz SSID, and a 2.4Ghz guest SSID for the IoT devices, which worked fine. But the router is inconveniently located as per today where the fiber modem is, so I wanted to lower the work temperature in general if possible and before considering moving stuff which would be inconvienient with several visible cables.
Therefore I hooked up said old N router on 2.4Ghz for IoT devices, since I didn't need that standard 2.4Ghz SSID at all either.
Speed tests from an ethernet-connected host to the AC68U showed up to 49MB/s reading a large file from a NAS (ethernet connected to the AC66U media bridge). But this was sped up as soon as I both deactivated all the 2.4Ghz networks and the 2.4Ghz radio in both Asus devices.
All steps:
1) Disabled all 2.4Ghz networks on AC68U from the GUI
2) Disabled 2.4Ghz radio on AC68 from the GUI
3) ssh / nvram powered off 2.4Ghz radio on AC68U and made changes permanent
4) ssh / nvram powered off 2.4Ghz radio on AC66U and made changes permanent
Bash:
nvram set wl0_radio=0
nvram commit
reboot
Before: up to ~49MB/s (~ 390 mbit/s)
After: up to ~70MB/s (~ 560 mbit/s)
Inbetween this discovery, I also saw that the CPU temperature of AC68U lowered from ~77-78 degrees celcius to ~70-73.