Sent forms to disenroll from part B two years ago, was never disenrolled, figured it wasn’t going to happen and started using it, now have gotten disenrollment confirmation

Looking for advice. I have Medicare through SSDI. I was awarded SSDI and Medicare about two years ago. I decided to disenroll from Part B coverage then because I didn’t think I was going to use it (I have private insurance coverage still from a family member) and it was pulling several hundred dollars from my SSDI check every month. I never heard anything after trying to disenroll. I figured the info got lost. In the past 1/2 year, I’ve used the Part B coverage a little bit - mostly just to cover lab tests. Then, just now, I got a letter saying I was disenrolled and would receive backpay for the period I had been enrolled. What’s going to happen now? Will I have to pay Medicare back for the claims that had been made in that time? Also, I don’t really want to be disenrolled at this point. The private insurance coverage I have will probably be ending about a little less than a year from now. Back when I originally tried to disenroll, I figured I could re-enroll in one of the “special enrollment periods” to avoid the penalty costs for not having Part B (I qualify based on the type of private insurance coverage I have). Is my best bet to regain coverage to try to figure that out now? I can’t just go haha oops, can we actually undo that- right?