Vigor2830n 3.6.4_sb_232201
Using the WAN2 connection on a plusnet fibre connection.

Download speeds were at 36mb when first installed but have fallen to 8mb ever since I got a fixed ip address from plusnet. Using both the BT and speedtest.net testers to confirm. Have logged call with Plusnet and they cannot find any problems.

Connecting directly to the BT box router with a laptop and I get 36mb and also using the plusnet router I get the correct speed. It seems to only affect my router after enabling the fixed ip address.
Upload speed is unaffected at 8mb no matter what router or method I use to connect to the internet.

Any ideas?

Go to Bandwidth Management >> Quality of Service and untick the voip priority ... also make sure the rest are inactive

Go to Firewall >> DoS defense Setup (quality of service) and set higher values, untick Block IP options & untick Block Unassigned Numbers ... or simply disable QoS

Go to WAN >> General Setup >> wan 2 >> Physical Type ... change it to 100M full duplex or 100M half duplex etc