How may I access the DHCP settings? Want to make printer a FIXED IP

The Nokia gateway nor the Arcadyan gateway appear to provide any static IP exclusions it seems. I have not had any issues with the printers I have here with dynamic addresses so I have not bothered to mess with it. All my devices have obtained IP addresses 101 or higher. That being said the only fixed exclusion to the DHCP range that I can see is the router IP .1 so you might be able to just make the static address setting to the printer below 100 and it could be fine. If you have an IP conflict with the static entry then you will know. I have a record of 24 device IPs for clients on my LAN and 15 to 16 active most of the time and none have an IP below 101. T-Mobile only officially supports 64 hosts on the LAN. The router shows 128, i know, but the official documentation shows 64. You don't need a scope larger than what is intended to support the client supported.

Ping the broadcast address 192.168.12.255 and then show the ARP table for the client. This is simple enough to do. With a Windows client open the terminal, PING the broadcast address, then issue the command arp -a and review the ARP entries the client reports. I run Windows, Apple, and Linux clients and all seem to respond when I ping the broadcast address so it is pretty reliable way to know what clients are active and responding to have a listing. You can get an application to run a ping sweep of the LAN to do the same thing. With a packet capture you should be able to see the lease duration provided. I have not bothered as I have not seen any churn in the IP addressing of the clients on the LAN. If it isn't broke I don't go fixing it.

I am pretty good with Terminal, I am on a Mac- how do I run the arp on there?

For macOS:

1. Open the Terminal App. go to Applications > Utilities > Terminal or Launchpad > Other > Terminal.
2. Enter the "arp" command with an "-a" flag. Once you enter the command "arp -a" you'll receive a list with all ARP entries to the ARP Table in your computer.

The responses are much cleaner on a Windows machine via the terminal or on a Linux client. I may have run my initial sweeps with a W10 client or with one of my Linux clients. If you issue a ping for a specific IP address and you obtain no ARP response then it is not an active IP address. The ARP response provides the IP to MAC address mapping. If you want to use 192.168.12.99 just ping that IP and see if you get a response to the PING. If not it is probably not live.

Another option you have is to install nmap from nmap.org. There is a free version you can download and install. The instructions are there on nmap.org for the MAC osX install. It is pretty simple. Sure the MAC will balk as it is not from the Apple store but it is a safe tool. Once it is installed you can run a scan of the subnet. for the "Target" you enter 192.168.12.0/24 -v (add the -v if you want verbose) if you want really verbose add -vv or just run the scan of the subnet with no option. Once the scan is done I like to select the "Topology" tab and then the "Host Viewer". You can always just select the Hosts on the left hand pane and see the discovered hosts from the scan. I just like tinkering with the tool.

Thank you for mentioning the DHCP addresses start at 101.  I recently changed over my network to the 5G gateway and had several devices at fixed addresses.  I had set my older router to also start DHCP addresses at 101.  Since the Nokia gateway seems to behave the same way, everything worked.  Of course, I did have to change them all from 192.168.10.xx to 12.xx.  I have seen no issues with any of my devices since I switched.

I originally tried to run the output of the gateway into my existing router, but the double NAT cut my speeds  (150/40) almost in half.

Great that it has worked out. I do NOT know 100% that the DHCP scope starts at 101 but the behavior I observed strongly suggest this to be the case. 

I have seen other users reporting using their own "router" with the T-Mobile GW. It is possible but of course you have to put your unit in to bridge mode or AP mode and disable the DHCP server. Then you could use your own wireless router. Of course I suspect then you would miss some of the features it would normally provide when it is in its native state. Some users used their own wireless router to offload work from the Nokia. Some users reported heat related issues with the Nokia and the theory is unloading processing from the Nokia helps. I have not seen any heat issues with mine so adding another wireless device and spending more money was not necessary. The 802.1ax capability of the Nokia seems to be fine. I have no 802.1ax clients to exercise it so not a big deal to me. I get good performance from my Nokia GW so I don't bother trying to fix what is not broken.