Some famous app are GMON and GNETTRACK, but this one seems a lot better:
Network Signal Guru
Excellent app to debug mobile network signal. Its shows band data, mimo and also modulation.
It is an excellent app for network information. Lots of detailed information on LTE signal quality, carrier aggregation, mimo, modulation, VoLTE and other related network information.
Biggest downside is the recurring monthly price for the full version which is way out of reach ($50/mo); Also a downside is root needed.
Without it, certainly the best!