RMerlin obviously meant flushing the browser cache, with CTRL-F5...
Also dont take it wrong, but if you dont yet know something that basic; flashing a modified FW to your router might have no point for you and even be dangerous (bricking it or just misconfiguring it etc.)?
RMerlin said: "If you have just gone from ROG to regular UI, I recommend flushing the cache as a few cached files (mostly CSS and JS) will be different." I'm sorry, but it didn't seem obvious to me that it was referring to the browser's cache. I have Merlin in my last 3 Asus routers and I also have a FRITZ!Box 7520 with openwrt. I don't have any problem flashing a modified FW. Thank you.