Well, i think you should email tp-link guy for help.

I think so.

Hum, if you have internet access, just leave the errors go. Maybe they are useless.

Make sure you can print wirelessly with the modem router by pass the WAP. And then you can set up WAP as Universal repeater instead of client mode.

I don't have this router yet. But i think chaning it to openwrt is not a good choice since it ruins the warranty. why not change the wireless channel when you encounter the dropping issue?

Maybe the firmware you updated is NOT for this router V3.