Why is data roaming greyed out

[Android Central Question]

I bought a second hand unlocked Samsung S7 in Europe and it had worked fine for a while. Recently though I can't get data to work. I currently have a Croatian sim card inside and can call and text fine but not connect to the internet. I am in Croatia and yes I have data on the sim. The phone is a Galaxy S7 SM-G930V and was once locked to Verizon. When I go into settings-connections-mobile networks I have selected allow data roaming access for all trips however the roaming option underneath is greyed out. It says "not roaming" and I can't press it to change.