Arcadyn IP Address

No, the established address range for the network is fixed. The deployment for clients is via DHCP. It is possible to configure clients with static addresses in the lower part of the subnet. From what I have seen client addresses via DHCP begin at .101 and range up to .254. The gateway IP 192.168.12.1 is fixed.

My home network is in a different subnet, not sure how to make this work.

So, are you saying you are intending to use your prior router with the T-Mobile Arcadyan gateway? Some users do but if you establish a double NAT solution you can encounter issues. 

You can put a third party router into AP mode and make sure its DHCP server is disabled. Then you would use the DHCP server of the Arcadyan gateway. Using another router with the T-Mobile GW will take a bit more effort. If you plan to use the WIFI from another router then you would need to go through the process to disable the radios on the Arcadyan. The best course of action then is to go watch Nater Tater's YouTube videos and gather more information. Others on the T-Mobile Community have used their routers or mesh solutions with the T-Mobile gateways you just have to work through the process. Those who have gone down that path have first hand info on how it worked for them and can make suggestions. 

You cant run a different subnet on the Arcadyan default subnet and expect it to work as you can't change the 192.168.12.0/24 subnet in that layer 2 broadcast domain and the 192.168.12.1 gateway address is set. If you are not real familiar with networking then you are going to have to do a bit of reading and research to make things work. 

If you want to make it more simple then you can migrate the clients to the 192.168.12.0/24 subnet and leverage the Arcadyan DHCP scope for clients to get their IP on the LAN. If you have clients on an Ethernet switch that is simple just connect the Ethernet switch to the Arcadyan and then have the clients release and renew IP addresses from the Arcadyan DHCP server. Of course you would have to get wireless clients authenticated to the WIFI of the Arcadyan OR if you use the third party router in bridge mode or AP mode then the clients can stay on the wireless radios of that router but get their new IP addressing from the Arcadyan. 

You can go down the path to use the Arcadyan with the wifi radios disabled and add the third party router just know it will take a bit of time and effort to make things work.

I'm fairly knowledgeable about networking and have some devices with fixed IP addresses.  Quite frankly, the Arcadyn doesn't work well enough for me to justify all of the work it will take to make it function as I want.  It's also too difficult to deal with T-Mobile's tech support as they won't let you talk with anyone who really understands the issues.   Will probably just return it.  Thanks for responding. 

You are welcome! The neighbors down the street that just moved here from Michigan could not get on the T-Mobile tower so they went with Verizion and the told me it was working well for them and they are happy with the solution. It all depends upon the area where the cellular signal cover and what obstacles are to be dealt with. Here for me I have a clean clear path to the tower and no obstructions and the tower is stable. It was a win for me.