I had to have our benefits (HR) folks talk to them. Unfortunately I don’t have that contact at Cigna. You can try with your certificate of coverage first to see if that is enough, so long as it specifies the necessary coverage levels.
You can do your test and go in Chiang Mai if your international flight flies directly into Chiang Mai (not through Bangkok). As of today, the only international flight into Chiang Mai is through Singapore. Hoping more will be added over time.
Don't worry about your passport number not being on your certificate. That's actually quite common for the U.S. CDC certificates, or whatever you have from the clinic that gave you your vaccine. Our certificate doesn't have our passport number, and it's not a problem for the thailand pass.
Just one international flight in from Singapore. So if coming directly from Singapore, you can do your Day 1 here in CM. Lots of options in town. Several friends booked with Stay with Nimman.