Hello @cheraloha, I'm not sure about the cold plunge but there are a few PMR related discussions on the use of infrared saunas. Here's a link to the discussions and member comments - https://connect.mayoclinic.org/search/?search=PMR+%2Binfrared+sauna.

Here's a discussion on the sauna use that might be helpful:
-- Any experience with red or infrared light therapy for PMR?
https://connect.mayoclinic.org/discussion/any-experience-with-red-or-infrared-light-therapy-fro-pmr/