Wireless Joint

I have a Galaxy S9+ with a botched screen replacement (oops). I was thinking about keeping it to tether with, but I'll need to upgrade firmware, etc, and the top half the screen, I can't see

Any chance I can I remove the modem from it & drop it in an enclosure or router? A cat18 would be nice

https://www.quectel.com/product/lte-a-eg18-na/

You would need a hot air station and a suitable module to put the modem in. That modem is soldered directly to the main board of the phone and all of it's auxiliary components are part of the main board. What you are basically wanting to do would be similar to pulling the processor out of a laptop (soldered in) and wanting to transplant it into a desktop (socketed cpu). You need more components and someone with the ability to play with the firmware to possibly get it to work.