It's the Three network in the UK, not an MVNO, and I'm paying for the "unlimited data" plan. They don't offer a higher tier as far as I know.
I've been using it every day for work for at least 10 years, and it served me very well, including all my remote work through the pandemic, gaming and Netflix, sometimes downloading terabytes per month without issues.
But they've managed capacity disastrously in my area recently. For the last three months. Where I live, and other parts of the centre of town, every day for about 4 hours from 11am to 3pm, it goes offline most of the time, so the network is unusable. It's impossible to work - can't even do chat or an audio meeting.
When I ran ping, I found it wasn't really offline sometimes. Sometimes it was just such high latency and/or packet loss that it may as well be. Signal strength was good, and voice calls worked fine.
I tried three different 4G/5G routers, four different accounts, and two phones. All showed the same behaviours at the same times of day. At first folks in the Three store said I need newer 5G capable equipment, then it was a new SIM, but I tried all their options. In the end I returned everything to them, cancelled contracts, and they said it was most likely congestion, which fits the observations.
I'm amazed they haven't fixed it, as it must have been affecting thousands of customers for months, in a way that's surely obvious to any monitoring equipment.
With regard to the earlier poster's point about latency, when it worked perfectly my latency (both at home and in the office) was always at least 35ms or so, spiking randomly on a timescale of seconds up to about 400ms. Good for many things, but not the kinds of low-latency gaming, interactive streaming or other services some people take for granted. SSH felt annoyingly slow, but usable.