what phone, what country, flyme 6 or flyme 5? i think it is the provider, try to change your provider as in my country there is no problem about LTE network.

m3 note, country-india, provider-jio
my meizu m3 note gives very poor network strength compared to other LTE phones at the same place.
my phone shows one bar of signal while other LTE phones show 4 or 5 bar of signal at the same place and same network

capture the log file and post in bug section.!