WiFi/Network problem

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Anyone else experienced this? I think it happens when I have WiFi enabled but set to switch off when screen is off. If I leave the phone like this for any length of time (say overnight) when I switch on again there is no GSM connection - only a full reboot will initialise the sim and get me back on to T-Mobile. Spoke to T-Mobile about it who fobbed me off to Huawei who said I should speak to T-Mobile. They are prepared to change the phone and claim there'll be an engineer in store to help which I doubt - and I don't want to lose my settings.

Any thoughts?