Thanks for the reply. I think I have found the answer.
In iOS 10, any app that would like to access network will have to be granted the network access right manually.
Normally, there will be a pop up dialog but in some cases, as noticed by many other iOS apps, that dialog will be "silent"ed. In this case, please follow the following steps:
1. In Settings, Apps that use network, check if "Resilo Sync" is listed, if yes, turn the network access right on.
2. If not, (and here is the tricky part):
2.1 Disable any listed app network access and re-enable;