@Domblewomble therecis a default setting on iPhones which uses mobile data in preference to wifi where it is faster. Check the phones settings apps go into the mobile data settings scroll right down to the bottom and uncheck WiFi assist which should help.
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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
