Fibre speeds vary according to the distance from your home to the cabinet.
What speed range were you given when you joined and what minimum guaranteed speed ?
What is your downstream line rate as reported by the router ?
The only reliable speed test is with one device connected to the router via LAN, not wireless.
When you get disconnected, is this from just from your WiFi or completely from all internet ?
When this happens what do the status light(s) on the router show ?
I'm sorry to hear this.
I've completed a line test which hasn't detected any faults and I can only see 1 re connection on the line.
Are you connected wired or wireless when you experience this issue?
What speed are you receiving when running a speed test?