Highly unlikely to be cell phones usage of the N77 low or high bands (3700 - 3980MHz) as these bands are primarily used for the 5G home Internet type services or micro nodes (smaller neighborhood coverage). We know which providers have licenses to use specific bands, but I don't know of any databases of what bands are currently in use in your region. Depending on the band pass range of the LNB or LNBF that you currently are using, the interference may be coming from other devices and frequencies and not at all associated with 5G. Wide Area WiFi is often licensed on frequencies around 3500 and 3650 MHz. Other RF licensed and unlicensed signals are in operation between 3400 and 3700 MHz.
The best way to identify the interfering signals is to use your current LNB(f) aimed at the problematic satellite and monitor the IF of the entire BPF. Example: If you have a LNB(f) with 3400-4200 MHz range, insert a 2 port 3GHz splitter with 1 port power passing connected to your STB and the other power blocked port connected to the spectrum analyzer. Set the SA range to sweep 950-2150 MHz and watch for signals popping up that correlate with the timing of the interference. Be sure to change the STB polarity during the testing, as interference may be stronger on one polarity and attenuated on the other.
BTW, there is no 5G band that would interfere with your KU FTA system, but there are many other terrestrial and low orbit satellite services that potentially could interfere with KU band.
