Normally, I have Wifi and data mobile both enabled. both adapters wlan0 and wwan0 have an IP and routing is like that:
pureos:~$ netstat -rn
Kernel IP routing table
Destination Gateway Genmask Flags MSS Window irtt Iface
0.0.0.0 192.168.178.1 0.0.0.0 UG 0 0 0 wlan0
0.0.0.0 10.124.31.240 0.0.0.0 UG 0 0 0 wwan0
10.124.31.224 0.0.0.0 255.255.255.224 U 0 0 0 wwan0
192.168.178.0 0.0.0.0 255.255.255.0 U 0 0 0 wlan0
where 192.168.178.1 is my AP at home. When I now leave my Wifi zone wlan0 goes down and the above first routing goes away. But IP is not routed through data mobile, at least not for a long time. I found no other way to switch to routing through data mobile than waiting for a long time or do a reboot.
The APN settings are fine and if I switch off Wifi, data mobile works fine. It does not work, if I leave any Wifi zone without switching off Wifi before. I.e. it does not switch by its own if Wifi gets lost.
I don’t have a Librem5 to test with, but maybe you could create an app/shortcut that takes the wifi interface down or demoves the default route that goes through the wifi router.
No need for this. One can pull down the above status line and tap the Wifi icon to switch off or on. But I think the L5 should do this by its own (as my Ubuntu mobile does), because you will not always think in this in advance and afterwards switching Wifi off does not help.
Mobian/Phosh does this, but not very well. Gnome Podcasts is not very frugal with bandwidth (constantly checking for new episodes) and lots of websites auto-reload (I have dozens of tabs open and forget to close the browser), so my mobile data was getting used up halfway through the month. I finally took to turning off mobile data at home because the wifi was connected, it would often route through mobile anyway. I wish we could turn off mobile data from the notification bar, like we can do with wifi.
On my Ubuntu mobile E4.5 you pull from the notification bar the four networks symbols (for the two SIMs, Roaming and for Wifi) and disable Cellular data by taping the green toggle switch:
