International roaming, forbidden networks, hidden sim toolkit on one plus 5T
I had same problem with Airtel but in another country. But same SIM worked in same phone in some other countries. When I used to land in one particular country it won't latch to any network. I was having this problem for almost one year. I managed by inserting the SIM in a feature phone and it worked well. Finally I figured out what needs to be done to make it work in a smartphone. Please go to manual network selection and select your operator even if it shows as forbidden. It worked for me and might work for other also.