How to change the Wifi and LAN IP subnet

You can turn the WIFI radios off by disabling the SSID for each radio and then just connect your access points to the T-Mobile gateway. T-Mobile does not provide a way to disable the DHCP server. I don't know if you have the Nokia or the Arcadyan gateway but the quest is a futile one you have embarked upon. Go and check out Nater Tater's YouTube videos. He has posted some very informative videos that are helpful. If you have the Arcadyan he has provided information that will give you the way to get more information from the T-Mobile Arcadyan gateway and also how to disable the radios. 

With the NAT on the T-Mobile gateway I do not believe you will be able to disable the DHCP server and I have seen no configuration options for altering the DHCP scope. T-Mobile wants to control the inside and outside NAT interfaces. They don't offer a tremendous range of advanced configuration options.