The Riemann Sum integration only updates when the source sensor changes state. Your source sensor never changes state.
A triggered template sensor should work (goes in configuration.yaml, not sensors.yaml):
template:
- unique_id: '3423sfsfas34adsadaeaffds'
trigger:
- platform: time_pattern
minutes: "/3" # every 3 minutes
sensor:
- name: Fritzbox Current Power:
state: "{{ 16.8 + range(-0.05,0.05)|random }}"
unit_of_measurement: W
device_class: power
state_class: measurement
The small random value between 0.05 and -0.05 added every 3 minutes should average to 0 over time and be enough of a state change to allow your Riemann Sum sensor to function.