I apologize for the delayed response. I did not receive any notifications for some reason.
Is this still relevant?
Regarding your setup for a redundant Internet connection, I suggest considering VRRP as a solution. However, I am uncertain about the type of broadband router you are using, and whether or not it supports VRRP.
RUT950 supports VRRP. You can find more information about it here.
Since both your incoming and outgoing traffic is through the wired interface, using failover functionalities may not be the best option for your case.
Alternatively, you may want to consider writing a script that will switch to using the mobile interface as the default gateway if pings via the broadband router fail.