Fixed Wireless - use my own router

Nope, sorry.

Why do they not allow this?

I can only give you a layman's explanation based on what smarter people have posted here.  Apparently the processing of packets on FW is CGNAT whereas "regular" Internet services and third party routers use NAT.  It's sorta like the difference between cable internet and AT&T Internet, they don't talk the same language.

https://en.wikipedia.org/wiki/Carrier-grade_NAT

Carrier-grade NAT (CGN or CGNAT), also known as large-scale NAT(LSN), is a type of network address translation (NAT) for use in IPv4network design. With CGNAT, end sites, in particular residential networks, are configured with private networkaddresses that are translated to public IPv4 addresses by middlebox network address translator devices embedded in the network operator's network, permitting the sharing of small pools of public addresses among many end sites. This shifts the NAT function and configuration thereof from the customer premises to the Internet service provider network (though "conventional" NAT on the customer premises will often be used additionally).

…

Carrier-grade NAT usually prevents the ISP customers from using port forwarding, because the network address translation (NAT) is usually implemented by mapping ports of the NAT devices in the network to other ports in the external interface. This is done so the router will be able to map the responses to the correct device; in carrier-grade NAT networks, even though the router at the consumer end might be configured for port forwarding, the "master router" of the ISP, which runs the CGN, will block this port forwarding because the actual port would not be the port configured by the consumer.

IN ESSENCE you have a private IP address behind an upstream public IP address, more common with cell tower internet which ATT Fixed Wireless is, operating on band 30… at 2300 MHz

I guess the more technical info (thanks @my thoughts) didn't light the lamp either.

It's not that they don't "allow" it, it's that the two technologies are different from one another and thus incompatible.  If you want something where you can add your own router then find a conventional Internet provider like AT&T Fiber or Uverse Internet, Comcast, Spectrum, etc.  However, I suspect you're on AT&T Fixed Wireless because that's all that is available to you.

If someone is on ATT Fixed Wireless they generally have access to satellite internet from HughesNet or ViaSat (Exede) and may have cellular internet from other providers the most common being T-Mobile and Verizon. 

https://broadbandnow.com/Fixed-Wireless-Providers

We've found 1758 providers offering Fixed Wireless service in the US. Below are stats on their coverage and speeds

That's strange, my question asking why they don't allow this was immediately following @tonydi 's first answer of "Nope, sorry."

Anyhow, thanks for the explanation.  I'm happy with the service for a few years now.  Perhaps it's improved now, but satellite I thought was less speed and reliability/uptime especially in inclement weather.

My real motive for asking this really was around improving the wifi on my property, not the actual internet speeds/connection.  I suppose the modem/router/switch I have from ATT (wifi 4 and wifi 5 on 2.4ghz and 5ghz respectively) can meet my needs, I think I just need better coverage. 

Right now I have an over-the-powerline access point out in my detached garage which works great for my security cameras.  But I also remote desktop into a mini-PC to do imaging of the night sky with my telescopes and each image is 120mb.  I'm forced to save them locally and then painfully wait 30-60 mins to wirelessly move them to my NAS each night.  Sure I could walk out there and put it on a thumb drive, but what I'd really like is for to save the images directly to my NAS but the speed is just not good enough.

I'm hoping I might just need a wired and more powerful access point rather than over-the-powerline which might help.

Thanks.

@tonydi Both your and my thoughts responses were after the OP asked “why not allowed”. Marking them as solutions moved them in the thread order so they look like they were posted before the question, but they weren’t. The OP posted that question after your first response but before your second response.

Arrggghhh!  Thanks @sandblaster that makes a lot more sense and I should have caught that.  I absolutely hate how this stupid forum handles this function.  Every other forum that has "solutions" leaves the original in place and puts a copy at the top.