Please ensure 4G is enabled on the device, please also check for 4G 5G availability in the area (link below), it may be that they’re trying to connect to 5G but it’s not available, in this instance please ensure 5G is disabled on the device and the connection will toggle down to a 4G connection.
O2 Network Coverage Checker | 2G, 3G, 4G and 5G Coverage