iPhone 6S Plus Sim issues

[Fuschnickens]

Hey all new here and having an issue with a new sim card. I think the card is most likely defective but wanted some other opinions. I have an unlocked iPhone 6S plus. I'm headed to London on Thur so I was testing a new 3 UK Sim. When I put the sim in all I get is no sim. I know the new sim is active. When I place my att sim in it finds it right away. I used the phone in Austrailia with an Amaysim Sim card and it worked no problems. Popped in my old deactivated 3 sim and get a "No Service" in the upper left. The new sim is the only sim the phone cannot read. When I put the new Sim in I go to settings ->cellular and cellular is grayed out. Am I correct in believing the Sim is defective?