who is your internet service provider? have you connect wired internet to the lan port of this AC1200? i think it is better to phone hotline support so they can troubleshooting for you.

if you are still having problems, it is better to contact TP-link email support to let them know about this issue.

try to reduce the clients connected and add DHCP reservation again.

it is actually required to fill in the ful website address

if your printer can connect wireless, there is no need to use the USB controller utility. You can do a reconnect on the printer, like on the phone, find wifi and pickup this tplink or the any name...

what is the current firmware now? change the mtu size to a small one like 1460? can you ping google using cmd command? reset router?