Trouble connecting Netgear M1 to ASUS 4G-AC68U

I have a Netgead M1 running about EE SIM card. I recently replaced it with ASUS 4G router, as I needed a stronger local network. However the ASUS is not recognising the sim card anymore. I want to be able to plug the the M1 into the Asus. I’ve tried setting the M1 up to work as a modem but using an Ethernet and USB connection the Asus just isn’t seeing it. I’ve tried following various ways to get it to work changing the IP addresses etc. I naively thought it would just be plug and play (more or less) and now I’m not sure if it is possible.

Any help would be much appreciated.