Being denied physical therapy due to having a secondary insurance I don’t use

So I live in Montana. I currently live in the larger city next to the Indian Reservation I grew up on and am enrolled in, but I live in the city off the reservation for work. I used to use Indian Health Service (IHS) but due to the long commute I decided to get insurance through my employer and not use IHS whatsoever. They are notoriously hard to work with even when you live there and use their facilities.

I recently had surgery. My primary insurance being through my employer and I was advised to put IHS as my secondary, even though they immediately denied any claims I had. Which is fine because I have good insurance through my work and they actually cover a lot of it and I pay the rest out of pocket. I’ve had no problems there.

But now I’ve been referred to physical therapy. I checked to make sure the office I wanted to go to was within network, and it was, so I call to make an appointment and they tell me that I can’t go there due to me having IHS. I explain that they’re my secondary and I haven’t used them for nearly a decade and that’s why I have my other plan as my primary. They tell me they can see that but they still can’t schedule me and I’m not allowed to go there. I even asked if there was a way to just bill my primary only and they say they can’t do that. I’m calling a different place tomorrow that I’ve been referred to but I’m worried about the same outcome and I’m running out of options.