I also use ASUSwrt-Merlin frequently updated to latest firmware. In my region channels 12 & 13 are legal & therefore appear as options.
Using channel 11 & 6 resulted in longer periods of connection, but still occasional disconnections. Once it took 5 hours to reconnect by itself.
Setting to Auto has used channel 2 or 3 & has resulted in several days with no connection issue.
The display has frozen twice after ~ 3 days. Re-power & the AG-Pro re-connects very quickly.
My key learning so far is only use wifi channels in the ESP active scan range, ie channel 1-11.
I’ve tried “legacy” setting, all bandwidth options, “disable 11b” , & more all with no impact to the issue.
All 2.4G devices are IoT & connect on wifi “n” with one exception connecting via “g”
Home Assistant keeps reasonable history & I can be certain the issue is only with the ESP device.
Interference with Zigbee is a possibility. I have almost 20 active devices, mostly on zigbee channel 25 (nearest wifi channel 13) & several on another device zigbee channel 11 (far edge of wifi channel 1).