I did try to forget the network on the iPhone completely and then reconnect, but it did not help. And as already said, other devices experienced no such issues.

I eventually ended up returning this one and getting a RE605X instead. The RE605X works so much better :)

We recently set up a TP-Link RE650 in our home in order to extend our Virgin Media router's WiFi. We connected it as per the instructions with the router's WiFi and then placed it in the desired spot...