4 units work together should be OK.

it's not recommended to update between them but you do update it successfully. so ,we can only observe whether it can keep work properly. It should be working well because it's still working properly...

is there WAN ip address on the router? You mean you can still get internet via the modem? try to restart your modem and router if the modem is still working properly.

never meet this~ generally speaking, the LED is orange, there is no internet. Since internet is still working properly, you may observer it to see whether there is any other issue. If no, you can...

tplink will be happy to own such a good customer like you:) Actually, I also have one tplink router, TL-WR841N which is working perfectly for me. I love it as well although it's a little old.

check whether there is WAN IP address on the router. If there isn't, you'd better connect to your modem and check whether it's working. It seems the router hasn't been set up successfully. Or try to...