You are aware how fucking easy it is to link metadata from devices, no matter what connection they actually use once you have access to a countries backbone, especially if you are a low income country with just a few exchanges, a service that solely exists with centralised servers outside the country and no privacy laws? It's extremely easy. Devices fingerprinting,etc. is a thing.

The only benefit of Signal would be the fact that the man in the middle has no idea that you what you actually write. But considering that it's easy for them to just claim someone has done something and punish them it's no help here.

There are designated messengers for situations like that, briar is one example. But Signal is a really bad choice here and Wifi doesn't change a thing.

…Wasn’t the conversation about SIM cards?