How and when do I select a network manually?

Do it when the phone cannot find a network although the settings are right: open network selection, turn off automatic, wait for the list, then try the networks one at a time.

Once connected, set it back to automatic later: staying locked to one network stops the phone moving to a stronger one as you travel.

We do not state which network a plan will use: that depends on the supplier's agreements and local conditions at the time.