Here o2 fails to even load a website despite having 4 bars 4G.
That sounds like cellular spectrum congestion. Your phone can hear the mast "loudly" so shows 4bars, but if all the channels are in use there isn't anything left to use.
O2 and Vodafone have 10mhz of spectrum at 800mhz - good for distance but means lot of people on one mast.
O2 are using the 5mhz of 1800 they have as an additional carrier.
Three has 15mhz of spectrum at 1800mhz - less range so needs more masts
EE has 20mhz of spectrum at 1800mhz - less range so needs more masts.
EE and Vodafone also have 2600mhz spectrum, but Vodafone seems to have only deployed this in central London.
The more mhz you have per person the better.... newer phones (iPhone 6s and later for apple users) can join two freqs together (carrier aggregation). EE has a lot of this, 1800+2600 and O2 and Vodafone are starting to do this 800+1800.
plusnet unlimited fibre
80/20 since 2 Jun 14 / Sync 6th Nov: 58,280/10,784 kbps with G.INP
16 years UK broadband (Since 1999 ntl:cable trial), Asus RT-AC68U & HG612 - BQM - Flash Speedtest - HTML Speedtest