Disable wifi and NAT on the TMO-G4AR device and use my own router.

What you are asking for is a "bridge mode" option on the Tmobile gateway but unfortunately it does not exist and there are no indications that feature is coming in the future. Your only options are:

1. Run your own router in AP mode but you lose most of the advanced features that you mentioned above, but at least you can control your wifi channels and bands.
2. Run your own router in DHCP mode which creates a “double NAT” layer but gives you full access to all of your router’s features.

I opted for #2 above using an Asus mesh system and haven’t had any issues with double NAT. However I don’t play online games and others in this community have reported various issues.

One expensive option is to buy your own gateway that supports bridge mode and go through some gymnastics with Tmobile support to convince them to support a non-mobile gateway. There is a YouTube channel from "natertater" and he seems to have figured out how to do this.

I saw Nate Tater's video for doing exactly that. However that involves using a "business" sim and paying per GB. Not an option. The TMHI sim with not work in a 5G modem router. It is designed to fail. Been there, done that. Put the sim back in the TMHI device and it works fine. 

Double NATing  fails to maintain DNS, even though I use my own DNS settings. I don't play games either. I just want to be able to use a web browser without WIFI. Most of my devices are on Ethernet (wired).  I've played with my router settings and it is a losing game. Where I work, ATT has provided a cellar backup to our firewall. I can fall back to any one of three sources for redundancy.

I've been to a T-Mobile company store. They only want to offer me another TMHI device which I already have. If you don't have a business license, nothing will happen. 

I've had no issues with Comcast's modem in the past using my router. I was hoping I could save some money with TMO. I am starting to wander whether or not, if it is worth the trouble.

Compos Mentis wrote:

I saw Nate Tater's video for doing exactly that. However that involves using a "business" sim and paying per GB. Not an option. The TMHI sim with not work in a 5G modem router. It is designed to fail. Been there, done that. Put the sim back in the TMHI device and it works fine. 

Double NATing  fails to maintain DNS, even though I use my own DNS settings. I don't play games either. I just want to be able to use a web browser without WIFI. Most of my devices are on Ethernet (wired).  I've played with my router settings and it is a losing game. Where I work, ATT has provided a cellar backup to our firewall. I can fall back to any one of three sources for redundancy.

I've been to a T-Mobile company store. They only want to offer me another TMHI device which I already have. If you don't have a business license, nothing will happen. 

I've had no issues with Comcast's modem in the past using my router. I was hoping I could save some money with TMO. I am starting to wander whether or not, if it is worth the trouble.

Are you setting DNS on the WAN or DHCP? Setting DNS on WAN is recommended as it will allow internal names to still resolve.

I set it on the WAN. Internal names is not the issue. It is resolving internet web addresses.

What else bothers me, is I see unknown devices on the TMHI device list. I am too close to too many neighbors. I wouldn't eliminate hacking attempts. I never had that issue using my own WIFI router. I could lock the MAC Addresses. TMO has not lived up to my expectations. I am considering other options. There are too many security risk in the wild. I see it on the firewall at work. Hundreds of hacking and probing attempts everyday. 

A workable solution to TMHI would be nice, if it could be had.