Keeping home router as main router using the 192. addresses, Setting up M1 mobile router. as ISP

Setup of the Netgear M1 nighthawk, mobile router (ISP) with the Netgear Rax20 (ax1800) home router.

I wanted to keep the home router, Rax20 (ax1800) as my main router and have it manage ip addresses,.. and keep them at the 192.168 etc., range.  And use the M1 mobile as my ISP. 

It conflicted and changed the gateway to 10.0.0.1,  all my connected devices, all changed over to the 10.0.0 range. (makes it harder for me to manage my ip cameras)

What would i need to do? In terms of Settings.

Do I connect to the Lan port or Wan Port of the Rax20? I used the Wan , which i think caused it. (10.0.0.)

What else. Dhcp, off , M1.  or no 

does subnet matter.  on any of the two.

Thanks, for any ideas. I wish i had a guide for this.