Just out the blue I have lost 5G on Lebara UK sim in my ZTE MC801A router. I have checked for any outages and none reported. Could Lebara UK blocked 5G on my sim as I use it in a router?
You'll find people here using TBs of data on Lebara without issues, so I doubt they've kicked you out from 5G for using 300GB.
I'd test with another SIM just to make sure it's not an issue with Vodafone's 5G. You can't really trust the status page, most of the time it will tell you that everything is fine even if it isn't.