@Gandoo First, you could check out the Wi-Fi network connection, then make sure you have turned on the Wi-Fi calling in settings of the iPhone. If it is the iOS system glitches, try out using professional iOS system recovery tools such as TunesKit iOS System Repair and others to fix it. If they all don't work, you may need to contact the carrier for suggestions.

@Gandoo Got the same issue. Gave the children 12hrs a day, but the Wi-Fi Calling is eating it up even before they are awake. And this is something of late. Never happened during summer.

Created a test profile with my own iphone to see what might be causing it.

@Daniel_D I noticed the same issue, and have recently upgraded my firmware to see if helps resolve this recent issue (that may be related to child enabling wifi calling on their phone).  I'd recommend you check & hopefully do the same for your model.

I am NOT impressed that the Deco app offered no new updates (1.0.5 Build 20201013 Rel. 64247), yet browsing TPLink website, I came across 2 newer builds.
https://www.tp-link.com/ca/support/download/deco-x60/v2/#Firmware: Deco X60(US)_V2_1.5.1_20210204

Generally have been happy with the X60 v.2's, but its inability to self update makes me loose a little trust in TPLink products.