LB2120 4G LTE Modem connectivity issue

Hi there,

I've just got the above to provided backup internet connection, as most of our systems are now cloud based. 

The hardware we have are (excluding the LTE): phone line  -  TP-Link TD-W9970  -  Netgear Nighthawk X6 (to give wifi coverage in the building)  -  computer. 

I've setup as per the manual for "Connect the modem to a wireline broadband modem", using the TP Link router as "wireline broadband modem" and it works as it should, automatically swtiching the connection when I disconnect the phone line. However, our VOIP phone system doesn't work. 

I tried "Connect to a router directly" (to TP Link router) but I can't find the modem on 192.168.5.1. 

I've not tried connecting to the Nighthawk, but I'm guessing this won't work anyway, as it needs to come through the TP Link to work with the VOIP system? 

Thanks in advance for any help.

James