Cheapest WiFi hot spot?

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.

Any ideas would be appreciated.

Might help to know where you are as solutions for Europe might not be applicable to north america and vice versa.

Oops.
Boston, MA, USA, Earth

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.

https://www.mintmobile.com/features/mobile-hotspot/

My iPhone
I have no sim card and I am using internet, email and WhatsApp using home WiFi ,