Question VoLTE issues

[1075dn]

I have a Sprint Galaxy S8 that I paid someone online to network unlock, however it’s still running the original Sprint firmware. The device officially supports VoLTE, but in Settings->About->Status->SIM status, IMS says Not Registered, and all calls downgrade to 3G. That’s going to become a problem soon since 3G networks are being shut down where I live (outside the US).
If I flash the Samsung U1 firmware and then change the CSC to my local region (with SamFW), will that allow IMS to register and thereby enable VoLTE?

Also, I ordered a B-stock carrier-unlocked S25 (I think originally Verizon). Will I run into the same IMS/VoLTE issue with a US carrier model, and would flashing U1 + CSC change fix it the same way? I'm sure plenty of people here where I live have US carrier-unlocked Androids, will they all run into the same issue?

(A little unrelated but is there a way to check battery cycle count which withstands a reset? I want to make sure the seller is telling the truth when he says it's not used besides for someone returning it within 14 days)

Thanks

 

[1075dn]

Update: I got VoLTE to work on my S8 without needing to flash or change CSC using the following trick!
Go to GitHub (can't post direct link, I get flagged for spam)
User: kyujin-cho
Repo: pixel-volte-patch
Issue: 136
Comment: 7th from top, user arsradu

Make sure to set APN type in all caps, and select the new APN

Do I have to do this every time I switch SIMs and get new APNs? If I flash the U1 firmware will it work by itself?
And will I have to do the same thing with my S25?