Update to the latest iOS version (15.3.1 at the time of writing).
After you've updated your iPhone, make sure WatchMate isn't in the background, and if it is, stop it. (On the iPhones home page swipe up from bottom, and swipe up on the WatchMate app).
Using the iOS Settings app (as you've done previously) connect to the XB-8000
Tap the circled i to bring up the WiFi information. Scroll down so that you can see the IPV4 address details. Take a screenshot and note the address of the Router. Scroll back up and make sure auto-join is enabled.
Start the WatchMate app and go the WatchMate app's settings -> Connected Device. Check that the ip address here is the same as the Router's address you noted in 3. If not, change it. Tap the ip address (4 numbers separated by 3 periods) , type in the Router's address and select save.