Broadband speed keeps dropping - can anyone help?

Hi all, I’ve been with Nexora for about 8 months now on the Fibre 100 package but recently my broadband speed keeps dropping to as low as 5Mbps, especially in the evenings. My setup is quite simple, just the standard router they gave me. I’ve tried restarting it a few times but no luck. Could this be a fault or is the network just congested? Any advice would be much appreciated.