Earlier quoted context omitted.
It's interesting you've found the WireGuard experience to be "seamless". I have a WireGuard VPN at home and experimented with always-on, on my Android phone. Unfortunately, my provider (EE, UK) throttles UDP traffic something rotten, and my normally great experience with 50/50Mb+ is severly limited to between 0 and 10 Mb making my phone almost unusable by normal standards. Does your LTE provider not throttle this way…
On the 2 SIM cards I have (Vodafone NL and KPN NL) they don't throttle, as that's illegal, but the plans have data limits (after the limit they just disable 4G for you) and perhaps they do some QoS though. Public WiFi I mainly use Dutch railways (NS) in trains which uses T-Mobile NL. You (or well, anyone, AFAIK) cannot use that to watch on-demand movies though. But I just have that kind of material synced up locally.…
I haven't contacted EE about it or tested other VPNs yet. I want to run WireGuard for various reasons so switching to OpenVPN might confirm they issue but not solve my problems (I don't run the VPN for the reasons in the OP)