What parts should I purchase to fix the cellular antenna?

I have an iPhone 11 Pro, the cellular 5g or lte is always down….”Wifi works”…I did all the restart, reset the phone and still doesn’t solve the issue. I removed the SIM card and it would only work for an hr or so and it would go back to no service or SOS.. I’m pretty handy on repairing iPhone I just don’t know what part to buy to fix this. Any suggestions will help a lot.. TIA