I can't explain it, just confirm that it is perenial problem.
My EE and now T-Mob connection sometimes just stalls, for no apparent reason.
I am about half a mile from each of 3 towers, but none in line of sight. Never could find out which one(s) I connect to.
Almost two years ago my 3G EE connection stopped working altogether, EE could not explain let alone help, so I stop paying the monthly contract fee, and they cancelled my SIM
(and kept sending me bills for further 18 months...!)
I switched to pre-paid (and cheaper) 3G T-Mob SIM, again sometimes it's just stalls, often re-dialing helps, sometimes just for few minutes, sometimes I just give up for the day..!
It's the same wheater I use the dongle trough router or in a laptop.
Don't think I can get 4G, haven't bothered to find out, still too expensive...!