Why is my 5G service so horrible in my home?

You could very well be in a fringe area. Even in areas of "coverage" there are going to be spots that just don't get what 5G supposedly has to offer. And coverage maps are not always as accurate as they indicate they are.

I have an iPhone 12 but opted not to change my legacy unlimited plan to take advantage of 5G as I don't really see that it has that many advantages for me.

Please check 5G network coverage at your location : https://www.att.com/maps/wireless-coverage.html

AT&T is rapidly expanding it's 5G network across US everyday. However, there is no ETA reported.

If your area comes under 5G coverage and still experiencing service issue, I recommend you to call the customer support at 800-331-0500 for troubleshooting and further assistance

What can I do to get better service in my neighborhood?

There really isn’t anything you can do. You just have to wait, the 5G network is still being built out. This is why I’m in no hurry to upgrade to 5G. My iPhone XR has gotten those fast speeds you see and better on LTE.

I just checked the coverage map and sure enough, I'm on an island of LTE and surrounded by 5G. What luck. I've highlighted the area I live in. Maybe they will increase the 5G coverage over time.

I guess what I want to know is why was my 4G service so much better then the 5G service ? I have a 5G phone which worked better on 4G. And I have not switched locations since it went to 5G. 

5G coverage in my area is still spotty so I just leave my phone on LTE all of the time. No connection or speed issues. 5G service is still building out in areas and the more folks who are using it, or attempting to use it in a give area, the more iffy the connection becomes.

This 5G offering is a complete joke.... It never works and probably never will.  Why is it the phones work so much better in Europe ?  USA some leader we are.  I guess its all about Corporate Greed and profits and market your way to these without providing users what they are told they will get.  (Edited per community guidelines) you can't return the dam phone they got your money already.  

5G does work well, depending on your phone, in a lot of areas. But it is still spotty in most, regardless of what the coverage maps indicate. They just show how far the signal can be detected, not the usability of that signal. All carriers do that.

Do you really NEED 5G? We get by just fine on LTE with our 5G capable phones. In-home, because the signal is variable and un-reliable, we use WiFi-C (WiFi Calling) which works perfectly.

I just did a speed test to see if things have improved since my original post in April 2021. What a joke. I got like 1.2 down and 0 up. Luckily I have wifi, but if it ever goes out I'm screwed. Not even sure how to turn off 5G and use LTE. Two years have passed and still no change. Crazy