APN problem on LG G7

[Artmuzz]

I’m in the U.K. and recently bought a secondhand LG G7 smart phone that has been unlocked from T-Mobile. Since it’s now unlocked I put in my 3 mobile sim and I am able to make phone calls but I cannot go on the internet even though data is enabled. I then get APN details from 3 mobile to enter and I save the new APN and restart the phone but I am still not able to get on the internet on the phone. Please advise.