@MrsChambers 

Hi, may I know if only the Deco Wi-Fi is unstable? or the MR400 WiFi is unstable as well?

MR400 supports OneMesh while Deco P9 supports Deco mesh, which are two different mesh technologies, that is why they are unable to build a mesh network, thus you have two Wi-Fi networks now. If necessary, you could change their network name and password to be the same. 

Note: For troubleshooting purpose, it is still recommended to use different names so that you could tell which is which easily.

If you finally confirm that MR400 WiFi works fine, only Deco Wi-Fi is quite unstable, it is recommended to start a new thread under Deco product for help.

If you still want one mesh Wi-Fi only, you could either disable the WiFi of MR400 and use Deco products for WiFi only, or consider changing the MR400 to a Deco 4G/5G products such as Deco X20-4G, Deco X50-4G or Deco X80-5G which are working with SIM card as well.

  @MrsChambers 

Normally Deco products should have a stable WiFi as well even though it gets internet from a SIM card router, so if indeed the MR400 works fine and stable, only Deco internert is unstable, you could try to start a new thread under Deco products for troubleshooting Deco network issue.