GT-AX11000 Pro & MB8611 Not getting full speeds

I was having crazy ping deviation using the XB7 and XB8. So I wanted to get my own modem and use my router, the GT-AX11000 Pro. I set my XB7 into bridge mode, hooked it up to the router and everything preformed great. Full speed, etc. But I didn't want to spend the $15 a month on it. So I figured I may as well get the MB8611.

So here's my setup:
MB8611 > WAN 2.5GB Port on GT-AX11000 Pro 
10Gbe Port on router > 2.5GB Port on Motherboard.

When I connect directly to the MB8611 I get over my full promised plan speed (1,300mbps) and it was consistent. but as soon as I put the router in between it only jumps to 1,100 momentarily and then just jumps back down to the 900's- always the 900's for some reason. 

I made sure my router's firmware is up to date and QOS is disabled.
I did a factory reset on the router. I tried power cycling the modem. Nothing is helping
Anyone have ideas? I really don't want to use the XB7 and XB8 as they weren't super stable and the ping was all over the place.