Help bridge arris BGW210 router set up and optimization

Looking for help on how to bridge the gateway and optimize some settings.

I am having difficulty getting in to the network settings since I complained to ATT yesterday and also had to create a new message board account:)

I originally signed up for fiber 100/100 but had to bundle TV with it since there would of been data caps without bundling. I do not have my cable box plugged in and it just sitting in my closet. Turns out that fiber is "in my area" but this was misleading as the tech who installed the equipment was honest with me and told me there would be no way I'd get 100/100 as he had to use a line about 500 feet away and my dedicated fiber line would pass through a shared coax and I basically am getting what I'd get from any other internet provider. Now when I look at my account it says I have internet 100/20....?

Anyway, to the more important matter. In my gateway settings it shows 2 open LAN lines with max down at 100+ and max up at about 25+. However it shows two open LAN's and the broadband is split between the 2 which makes my max down around 50 and up at 10 (I only get these speeds when testing on ATT's recommended speed test site 🙂 However, in the real world I'm getting dramatic changes from 40 down and 8 up being the max with the average being around 30 down and 5 up. At times I'll drop below 20 down and 500kbs up (yes kbs!).

This is with 1 LAN connected and wifi turned off on the gateway.

How can I bridge, pass through to my own router, and turn LAN 2 off so my bandwidth is not split? Any help, optimization setting recommendations or walkthroughs would be much appreciated. 

Also why can't I use my own Motarola gigabit modem and why does everything have to be gateways?