How about only connecting one device to the router? if you are still unable to login, you may try a factory reset

USB printer, right? compatibility issue? you can check the compatibility list on tplink website: http://static.tp-link.com/resources/compatible/Print-Server/Print_Server_Compatibility_List_160804.pdf

It should be better if you can upgrade the firmware to the latest. The current firmware 20150514 is very old. Go to this link and you can see the latest firmware is 161008:...

The T4U supports adhoc network at hardware level, you can set up ad hoc on your computer, google "how to set up ad hoc on windows 10 computer" will find solution.

That's cool, hope my router works like that as well:-)

Is it the latest firmware? When the wifi drops connection, how about the wired connection? And can you still log into the interface of the router? Do you need to reboot the router to get it work...