[Solved] New R7800 gets progressively unusable

R7800 works normally quite ok with OpenWrt. Sounds like your settings are somehow screwed.

I suggest that you flash again, without saving settings. And then manually reconfigure all settings from scratch again. Do not restore form backup...

Alternative would be to login with SSH, use the command "firstboot" to reset things, reboot, and then manually reconfigure everything from scratch.

Currently probably the "master" is slightly better on 5 GHz wifi, although personally I have not had trouble with 19.07.

I have regularly published a "community build" for R7800, which contains some essential add-on packages. You might try that.