I'm going to correct you on your theory as it is actually wrong. IHD devices have never used either the mobile phone network or WiFi for communicating with the meters. Communications to the DCC from the meter via WAN on SMETS2 is by means of a cellular network facilitated by O2 in the southern half of the UK and by Arquiva using Long Range Radio in the northern half. Communications between the meters and the IHD is a completely separate beast using Zigbee secure communications on 2.4GHz. This was built into the standard right from day one as part of the first iteration of SMETS and has remained unchanged through to the latest SMETS2 and upcoming SMETS3. Earlier SMETS1 meters used an older version of Zigbee radios which had a slightly higher ERP and therefore a slightly better range in some cases, plus there weren't so many smart meters competing for channel spacing in the same location.
So your meters or IHD's have never used WiFi, and the SMETS standards explicitly prevent WiFi access to the meter. Although the WAN uses cellular, the HAN doesn't.
So your IHD not communicating since the meter upgrade is either because the IHD wasn't correctly paired to the newer communications hub, or the lower ERP of the newer Zigbee module means the signal just can't reach your IHD. There are no 'booster boxes' or repeaters for secure Zigbee. If it was a remote software update, chances are the new software wiped any pairing data.
And if your meter is made by EDMI, then they won't connect to DCC's networks either. They can't and won't be migrated onto the DCC networks.