I guarantee you that they didn’t say “we don’t know how to fix it”. 100%. And to suggest that they don’t know what they’re doing when the App works (mostly) fine with every single network except one should tell you where the problem lies.
There’s a simple fix to this. Incidentally a fix that O2 had to come up with. You need to use a VPN, that’s it. Works 100% all the time on iOS and Android phones. The likeliest culprit is O2 blocking ports that the app requires to run. A VPN gets around that – O2 forums are full of people who confirm this, and the App works for them once they activated the VPN.
You can argue that you can’t be bothered to workaround the problem, but the fact of the matter is that it’s not Mercedes screwing up, but O2.
Prior: SEAT Ateca Xcellence Lux 1.5 TSI DSG MY19, VW Golf GTE PHEV DSG MY23
Current: Hyundai Ioniq 6 Ultimate
Next: we'll see what's available in 2028.