The wires in the heated windscreen can really impact GPS reception - the signal is very weak coming from a tiny satellite 200km away.
Try and position the phone as near glass as possible (metal van body will screen the signal) and ideally a side window.
Also in most phones the GPS antenna is near the top so try and orient that closest to the signal.
Another thing to try is to start you GPS software outside the van. All GPS receivers have to lock and and synchronise with the GPS signals to get a location. They are deliberately designed so that if the receiver knows roughly where it is it will lock on faster, whereas a true cold start and working out location from scratch can take several minutes. While a GPS is locking on it is much much more impacted by signal strength so you may find it never locks on in the van but will be able to stay locked on if it's already found the signal outside. This can be why people's phones behave differently as it depends what your phone was doing before
If by phone phone provider you mean change the model of phone then that's one option if a rather last ditch one. If you mean change the phone network that won't make a difference as it will only effect which network the 4G radio uses, the GPS is entirely local (well apart from an AGPS location hint from the network to help it lock on to the locations faster)
