Check VoLTE provider support , if you manage to get VoLTE to work you are probably fine and can keep using your device. Your best chance to get it to work is reflashing to the latest Android so you get the latest blobs (baseband etc), if VoLTE works under Android it will hopefully work under SFOS as well and you can flash back to SFOS.
Before switching providers it might be worth finding out what is going on in swedish networks, my guess is that soon enough all providers will make that same move and stop supporting <4G devices. quick google gave me 2G and 3G networks are being shut down – some mobile phones may not be able to call 112 - Krisinformation.se
Yes it sucks. Sweden is by far not the first country to do that, maybe one of the first in Europe. None of this is new but it has been ignored in SFOS for way too long.
Good timing though for xmas phone sales.