Came back dy, happy to report that U mobile was absolutely flying in China.  Most place with full bar 4G+, which is already very ok for me.  And as we are using data roaming, no China's Great firewall block on FB and whatsapp etc.

At least for all the places in Yun Nan I visited.

You may bring a bit, I reckon about RMB 1k per person will do, most people or outlets have either Wechat QR code or Alipay QR code, and many have both.  So you may use TnG ewallet to pay under Alipay.  For wechat pay you need to reload certain amount before you can use it, if not mistaken.