Nexus 6P - No more signal

Hi, I've been to 5 different countries in the last month and, after landing home in Amsterdam Schiphol, my Nexus 6P said "No signal". No possible to receive phone calls, sms or have 4g connection.

T-mobile substitute my simcard with a new one. The sim card works fine in other phones but my Nexus 6P.

I have cleaned the partition cache and did a factory reset. Still no luck.

The technician said: "It's because you changed to many countries and the Nexus went crazy! We need to change the motherboard!"

I think it shouldn't be a motherboard problem, as it was working fine 10 days ago.

Any other had this problem?

Many thanks,
Chris