@DC53 there is a setting on many mobiles that will switch to use mobile data if that is faster than the WiFi connection. On iPhones it is called WiFi Assist and is near the bottom of the Mobile Date settings so you have to scroll to find it Turning that off may sort your issue.
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65inch Sky Glass, 3 Sky Streaming Pucks, Sky Ultrafast + and Sky SR213(white Wifi Max hub) main Wifi from 3 TP-Link Deco M4 units in access point mode
