Can attest to using this app often to help with signal issues until EE gave me WiFi calling so I stopped using it. My signal is awful at home for most carriers but decent everywhere else due to unfortunate location. If you use the app you can try some of the bands below even if you don't have poor signal as it could increase download speeds.
EE operates 2G on 1800MHz, 3G coverage on 2100MHz, 4G coverage on bands 3, 7, 20 but also 1 & 38, 5G on NR band 78.
O2 operates 2G coverage on 900Mhz, 3G on 2100MHz and 900MHz, 4G on band 20 but also 1, 3, 8 & 40, 5G on NR band 78.
Three offers no 2G, 3G on 2100MHz, 4G on bands 3 & 20 but also 1 & 32, 5G on NR band 78.
Vodafone operates 2G on 900MHz, 3G on 2100MHz and 900MHz, 4G on LTE bands 7 & 20 but also 1, 3, 8, 32 and 38, 5G on NR band 78.
Typically band 20 gives better signal due to it being a low frequency (800MHz) band which can travel further than high frequency bands and penetrate walls easier for better indoor signal. Not all phones are compatible to recieve every band posted above and also the additional bands aren't deployed at every location so often that's a bit of trial and error. You can Google what bands your handset supports.
If you have a SIM card provider that's not directly with EE, O2, Three or Vodafone you'll need to Google which of the 4 providers they piggyback off and also what they offer as some don't offer everything that the operator they piggyback from does.
This may have changed a bit as I've read about mergers and carriers buying up new frequencies but probably not too drastically as It wasn't too long ago I noted this down.
