Using a serial interface to the LTE modem on an MR6400

I have an older platform with a 3g modem in it that I need to replace with a 4g device as 3g is being shutdown in Australia. I am looking at a Tp-Link MR6400 and wonder if the ser2net will work with this device.

Hi, welcome to community, ser2net package is still present in OpenWRT.
Check the hardware revision of MR6400 before buying, not all are supported.
Modern way would be SSH and picocom to talk to serial-capable modem, or adb to more complex ones.

I have managed to get ser2net working and sending/recieving SMS messages. However I get NO Carrier when I try to voice dial, I am guess this maybe a limitation on the qualcom modem

You need to find their at command guide, they have one of 3 network modes active at any time, then diag read device then serial control device, which should be one for sms
https://openwrt.org/docs/guide-user/network/wan/wwan/ltedongle
No support for your router but this fork may have that sms code hidden in their github: https://ofmodemsandmen.com/index.html

I have the SMS working with the AT commands, but unable to get it to make a voice Dial, I think that maybe related to no VoLTE

You will get much better assistance at rooter fork of openwrt forums. They support exclusively mobile modems and routers with those.