If you have installed OpenWrt on your TP-Link MR6400 V5 WWAN LTE router, you would have noticed that the WWAN functionality does not work out of the box because it is not configured, functionally turning your $80 router into a $10 router.
It is better to make a custom build and flash it, rather than messing around in the web interface and over SSH. This way, this configuration survies a factory reset.
- Head over to the OpenWrt firmware selector
- Find the exact router you have