My understanding was that SMETS1 meters (which is what my meter was when I moved-into my current property) used the mobile-phone network, as did the IHDs. Originally, I was able to use my IHD in my property, and the signal was strongest near a window -- which would certainly suggest use of a mobile-network. But for SMETS2, the IHD only connects to your smart-meter, and the smart-meter now uses some private-network operated by a Government-founded body called the "DCC", which energy-companies can contact, but no-one from the public.
Since the SMETS2 update of my smart-meter a couple of years-ago, my IHD no-longer functions as my flat / apartment is on the third-floor, and all of the meters are in a communal area on the ground-floor. I understand there is another body setup to find solutions / workarounds to this, called the "Alt HAN" group, which, again, members of the public are prohibited from direct-contact with.
If anything is incorrect there, feel-free to correct me, BUT -- I would argue, politely: should I really have to know any of this as an end-user anyway?
The whole-point of smart-meters is they are supposed to offer two clear benefits to me as an end-user: (1) meter-readings getting submitted automatically; and (2) that I can use an IHD device to see my live energy-use, and historical data. Currently, I can do neither. So the main point here is that I'd like both issues to get sorted. I shouldn't really have to know all the technical-terms and appointed-bodies and white-papers and so-on... it should just work!