Tenda 6804g 300mbps

I have a tenta 6804 g router in my caravan , it has 2 devices connected to it ( beok thermostat and smartlife device ) it goes offline at least 3 times a day but then comes back on after usually 30 or so minutes . But I have discovered if I ring the number of the sim associated with it it comes straight back online immediately, any ideas ??.