You can also download the My Network app and report a problem, if there are any: https://www.o2.co.uk/apps/my-network
But as it states it should be automatic and seamless unless you have a 2G/3G only option.
Although if both are available, the phone's going to stick with a poor 4g signal before retraining to a lower-class 3G... Switching to 2G/3G option from 2G/3G/4G option or setting removes the congested 4G signal from the equation altogether, resulting in an immediate performance improvement.
If a simple tweak works, use it!
Track usage, upgrade and get support with the My O2 appDownload My O2