Cellular front end diagnostic

How do you guys go about diagnosing Cellular front end issues on iPhone? I’ve replaced the charging flex on two iPhones recently and they end up failing the Apple diagnostics after. Cellular appears to be fine and working. The replacement flex is OEM pull. It appears to be working correctly as it all passed the diagnostic except for the Cellular test. Is there anything specific I should look for? I cleaned and checked all the grounding contact points on the screen/frame and the pins themselves and they looked fine. The only repair this 13 Pro Max had was a charging port replacement. I only removed the lower components needed to do the replacement so Im not really sure what went wrong.