You can have excellent coverage/signal strength indication on your phone and still have zero connectivity as the base stations broadcast a special pilot/beacon channel solely for the purpose of giving mobiles a stable signal datum for each cell.
You can sit within rock throwing distance of a cell site, but if it has no spare radio resources, it can’t give you a data pipe to send all those lovely 0s and 1s to you through.
O2 has particularly poor capacity despite having decent coverage for a number of reasons.
1. It has the 2nd lowest amount of frequency spectrum in the U.K. after Three*. Lack of frequency = lack of capacity.
2. The Spanish owners of O2 (Telefonica) have consistently under-invested in network upgrades and have a nasty habit of funnelling profits back to Spain to shore up their ropey home network.
3. Having the customers of 4 MVNOs (giffgaff, Sky Mobile, Tesco Mobile and Lycamobile) all using O2’s network resources means that O2 has a very congested network.
* Three “gets” away with not having old 2G spectrum to migrate to 4G by having a network sharing arrangement with EE.