RBK852 in Router Mode Internet Slows to 100MB up/down

Hi team

I have had the 852 for a while now always had it behind another router and in AP mode but I wanted to use the VPN function of the Orbi. So I changed to the Orbi set it up in Router Mode. Got all the MTU settings from my ISP. It works fine for a while at full speed (850-950mb down and  500mb up). But every so often it just drops to 100mb both ways.

I did have this issue before, but decided to go back to the AP mode and use my router.

But now I need the VPN for longer option I wanted to move back to it.

I updated the Firmware to the latest V4.6.14.3 Then I factory reset my Orbi Router and then set up fresh without copying settings and twice in 1 hour it has dropped from full speed back down to the 100mb both ways.