Welcome to the forums
@Selk.
Flipping the settings back and forth has been known to put the router in an unstable state. It is not equivalent to just leaving the settings at defaults in the first place.
What routers are you using? If they're both Asus routers, use the link below to fully reset both of them.
After you have successfully flashed them to their latest and most current firmware.
[Wireless] ASUS router Hard Factory Reset | Official Support | ASUS Global
After the routers are both reset fully, turn off the RT-AX55U. Then, physically unplug it from both the AC wall outlet and the router too.
Use the link below to properly set up the main router.
Fully Reset / Best Practice Setup / More
After the main router is fully set up, let it settle down for 15 minutes or more. Then, reboot it via the GUI. After a further 15 minutes, you can turn on the repeater and configure it.
I would suggest you use new SSIDs (at least to test with). Leave most things at defaults (as outlined in the Best Practices link above). Do not make too many changes all at once after you get the network stable and working as expected.
With the above completed, I would be introducing client devices one at a time to the network and testing that it continues being stable and responsive too.
Keep good notes on all the above steps. They may help you/us further troubleshoot things if they once more go awry.
HTH