I have a project that would work really well with a mobile WiFi hotspot, but I don't want my phone to be that hotspot. (It would be using only a few Kb per month). And I don't want to buy a hotspot from my phone provider.
In Europe you can buy 3g/4g routers for about £50 upwards, and you can get data only SIMS from about £2 /month. Not sure if you will have the same options. There are some cheaper router options on Ebay and you might be able to find a used one if looking for a lower cost option. Is it feasible to use an old mobile for nothing other than a hotspot?
Some resellers of Big Data allow free hotspots... an old cellphone works great for this setup. Monthly cost is not too bad at $15 USD for 3GB data/mo on 4G/5G. Economics really works best when you must have remote control of the node and cellphone can be used with DTMF decoder for management.