We're on the same wavelength here. I made a similar request over two years ago. See FEATURE REQUEST - Probe with source-interface. "Brand C" has had this for at least a decade.
You can come close with a route-map as suggested in Configuring WAN Failover with Network Monitor in AOS , but it really doesn't work 100%. If your primary ISP keeps your dynamic address within the same subnet, then you can indeed configure a route-map as suggested pointing to the (static) gateway. If, however, your primary ISP ever dynamically changes the entire subnet assigned to you including the gateway then you're in trouble and things will break badly.
I may have to engage my Adtran sales engineer to make this work. I suggest that you do the same.
In the meantime, if you want it reliable, your only real choice is to get a static IP address from your primary provider, and many will charge extra for this.
Like you, I've seen this option in other brands for some time and thought I was likely just missing it here. I was seeing the option here for selecting an interface instead of an IP at the key junctures (Next Hop Interface and Route Table/Gateway Interface), but it was ignored as option in the guide and didn't seem to work when I tried it--maybe it's in the GUI for future use? Thanks for clarifying the issue!
You're right, we were trying to avoid the extra charge for static IP for my customer, but it looks like it's either that or choose different equipment, for now.
I'll run it by our sales engineer and see what they say, and point back to your long-standing request.