The good news is internet works. Which firmware you've updated to? And what's the original firmware version? Can you share a screenshot of that "invalid value"?

The driver on the tpink website is not compatible with Mac 10.12, it says Mac OS X 10.7~10.11. I heard there is beta for 10.12. contact tplink support to get it

It will show you the used data usage on the app or the web page of the product. http://www.tp-link.com/faq-1132.html

The APN is provided by EE as well as other info such as authentication type, username and password. You should contact EE to get it.

The USB port on the M7350 is for you to access the SD card files or get internet on your pc after driver installed. I don't think it will work as a regular modem. http://www.tp-link.com/faq-758.html...

Your router is a US version or EU version?