ppp as the primary interface with wifi as the backup

On Ubuntu 21.10, I have a cellular modem connected via USB that I need to be the primary interface on which all network traffic utilizes. However, I also need the wifi interface (in my case, wlo1) to also continue to work, and be the backup in case the ppp0 interface goes down. ping google.com -I ppp0 -> works through ppp ping google.com -I wlo1 -> works though wifi ping google.com -> works through ppp (this is the difficult part) I can get the cellular modem online just fine using pppd and corresponding scripts, and get a valid IP address. It's the routing I seem to be struggling with. What I've tried =============== pppd ---- I first tried utilizing options in the PPP script, options like defaultroute and replacedefaultroute. defaultroute does not alter the routing table and add a default route at all. Perhaps this is because, as I've come to learn, a routing table can only have one default route. Not sure. When trying to use the replacedefaultroute option, pppd does add a default entry into the main routing table, however, this winds up breaking both interfaces and I have no internet connection whatsoever. It does not remove the existing default route for wifi, thus there exist two default routes in the main routing table. Manually adding routes ---------------------- I then played around with adding routes to the main routing table manually. Whan I run sudo ip route add {ipAddress} dev ppp0 src {ipAddress}, then both interfaces work, but wifi is still the primary interface. Attempting to add that route with a metric lower than the wifi's metric also does not make it so that the ppp interface is primary (i.e. ping google.com still goes out via wifi). I can add a second default, but it must have a metric *higher* than wifi, else it causes both connections to stop working. A second routing table ---------------------- Following other stack exchange posts and articles like this one , I explored the option of having two routing tables and adding a route and a default route the second routing table. While this doesn't break either connection, even using a metric lower than the wifi's metric, the ppp interface still is not the primary interface. Also, if I disconnect from wifi, then the ppp interface stops working as well, and the 2nd routing table is cleared (is this NetworkManager doing this?) Questions ========= Is what I am trying to accomplish even possible? If so, what am I doing wrong? Am I fighting with NetworkManager? Any help is *greatly* appreciated!