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Hardware Version :

Firmware Version :



I'm using the TL-MR3420 router with the ZTE MF823 USB modem.
I'm using a brand new 4G sim card from Three UK.

The modem works fine in a windows 10 laptop and automatically gets a connection when plugged in.
The router see's the modem fine (shows "Identify successfully" in the web ui of the router). Its also in the compatibility list.
I choose the following:

Location: United Kingdom
Mobile ISP: 3
Connection Mode: Connect Automatically
Authentication Type: Auto

The router never connects to the internet.
I have the latest firmware as well.

Can anyone help me with this?