If you are a fairly frequent traveler, you should get a credit card that doesn't have foreign transaction fees. I don't have any problems using the Didi app with any of my credit cards, except I think they don't take AMEX. What is a problem is linking a non-Union Pay credit card to WeChat. Also note that there are some non-Chinese banks that offer Union Pay cards, but from what I've read on this sub-forum is that not many have been too successful with these. Linking a VISA/MC credit card with WeChat will enable the wallet, but won't allow money to be transferred from the card to your wallet. For this it seems there are only 2 choices, open a Chinese bank account with debit card or give someone cash and have them transfer to your WeChat wallet.

I use the Didi app (not through WeChat) all the time and have it linked with a US bank issued VISA card.