That all makes good sense. I’m very close to a city to it’s unlikely I would be connecting to a 3G network (the same SIM in my other device, same make and and model but with stock Samsung OS, no changes) works just fine.
I have never seen my /e/ phone list 4G at the top right (though when it was working just fine I didn’t think to look). When I reboot the phone, it shows ‘LTE’, briefly (it flashes twice and then doesn’t appear again). Sounds like you’ve zeroed in on the issue. It’s not using VoLTE.
Doing a quick search on the forums, I see you’ve been helping people here with exactly the same problem and that as long as I’m using this Galaxy S10 (or any newer Samsung?) I’m not going to be able to make calls using /e/.
If I’ve interpreted it correctly. That sucks 
but what can you do. Get mad or get a new phone. Been thinking about a Pixel anyway…
Thanks for your help, this community is great, even if Samsung is not