Missing upgrade

No 4G or 5G. Why is that when I had it on O2?

Is your 'Preferred network type' phone setting set to 'Automatic (3G, 2G)'?
Settings > Manage SIMs > Tap on Lebara > Preferred network type > Mine is set to 'Automatic (4G, 3G, 2G)'
This means it will search for 4G as well as 3G and 2G.
There is another setting 'Automatic (5G, 4G, 3G, 2G)' but I don't choose this to save the phone battery. It will spend more time searching for a 5G signal that I don't need but you might prefer the 5G.

ThomasM15 some great advice from Wiganken here! You can also look at the network status checker to see if there are any issues in your area.