I'm not sure why you would want to use only the BT when you are at home. If the WiFi doesn't reach then there's a fair chance that the BT won't reach either. Fortunately once the cooker is programmed it just goes along its merry old way unless there is a problem (or unless there is a glitch when the app reconnects to the network and the cooker's timer gets reset).
I often use my cooker off-site where there is no Wifi network. BT connection is automatic if there is no Wifi network available. However, upon return to the home network it may be necessary to go through the network connection routine again. Not sure if this is an intentional thing or just the result of dodgy Australian rural internet connections.
For this reason I have taken to using only manual control on the cooker when using it off-site. However, it must be noted that temperature control when setting it via the manual wheel is rather coarse with only 0.5C increments available.