Want to use own modem with AT&T fiber

you must use the at&t provided modem, there is no other compatible modem

Here's that list:

Oh, right, there aren't any. AT&T only uses a handful of Gateways for residential-style fiber.  If you have to have a separate modem for AT&T Fiber, you'd have to order dedicated business fiber.

Well, frankly, this (Edited per community guidelines). I'm a quite certain that the slow service on some devices is due to too many of them connecting to the gateway.

That could be true on a Wi-Fi perspective.  As far as routing, it predominately matters how many packets, and what size they are, not how many clients.

What devices are slow?

Are they Wi-Fi?

Are they using 802.11n or 802.11ac / 2.4 GHz or 5 GHz?

Have you thought about adding your own router and putting the Gateway in IP Passthrough mode?

Have you thought about adding your own access points to help with Wi-Fi?

Thanks JefferMC. I have 3 AT&T Wi-Fi extenders and I have noticed that the devices the end up being slower are due to there being, for example, 14 on the access point, and 1-3 devices on the extenders. Sometimes a channel scan and/or restarting the device near the desired extender makes said device take the strain off the access point.   

Since all of the communication eventually goes through the Gateway, you can't really take the strain off of it.  What you're probably seeing is that they are too far from the Gateway for a good signal and work much better at the extender because it is closer.  The extender then sends the signal to the Gateway.

Most likely it’s your extenders that are the problem. I have 27 devices connected to my gateway. Wired and wireless. No speed or connection problems. Bgw210 downstairs. ASUS Ac86u upstairs as a wired access point. 

I have the Smart Home app and that is where I noticed that some of my devices weren't getting a signal. I then looked up how many devices were on the gateway and extenders. The devices with the orange WIFI coloration were connected to the gateway despite being in the same room as an extender.

So what did you do?