have you tried binance already? it appears that they both have usdm and usdt on the exchange it might work I personally have not tried converting usdm to usdt but you could check and might try if its worth and applicable , and besides there is nothing wrong in trying .
There's absolutely nothing wrong if we want to make attempts to solved the intending problem at hand, everyone can freely dropped solutions which can either succeed or not. The conversion have 50/50 chances of either succeeding or failing, though we have to keep positive mindset whenever we wants to gamble. The problem of swapping, we just gave to deal with swapping USDM to USDT and I'm confident this incident can be resolved. Trying Binance is a good welcomeable idea because he should keep making findings and following an appropriate steps of ensuring of solving issues.