The AVM Fritzbox (https://en.avm.de/products/fritzbox/) does not offer monitoring capability though SNMP or other standard monitoring technologies.
Can I monitor the bandwidth usage of the DSL, ADSL or VDSL Internet connection using LibreNMS?
In PRTG it seems to be there is a way with a native [EXE/Script Advanced sensors]
https://www.paessler.com/manuals/prtg/exe_script_advanced_sensor
Kindly regards