Still, the logs would provide valuable information about the issue. Wireless settings :

Check if the router overheats. You can search through the logs for more details during the reboot event.

@geekfer Try with these Wi-Fi settings on 2.4 GHz band and check if that helps:

You have Smart Connect feature enabled. That is why you don't see separate setting for 2.4 / 5 GHz bands, i.e. your both bands are with the same SSID and password right now. If you want to have them...

@TimZ Go into wireless settings and click the red circle to enter into 5GHz settings :

I've seen FW releases for AX-series routers, with description of bugfixes regarding compatibility with Samsung S10, S20 etc. (AX capable smartphones). OFDMA was enabled on AX20 when tested with...