I have a RUT240 and I got it to work, but not ideally.
But first thing you need to do is to change the WAN2 interface from auto to 1Gbps FDX.
The only way I could get it to work is via yet another bridge. In this case I have a Netgear LB2120 (which I bought the RUT240 to replace because it's rubbish but it actually worked well with the UDM Pro - I'm sticking with the RUT240 because it's got a better signal reception than the LB2120).
I tested all the combination: I could connect to RUT240 as "bridge" or "passthrough" and they all get an IP address when I connect with my laptop.
When I connect the LB2120 to the RUT240, the LB2120 gets an IP address (the LB2120's WAN address) too. Then when I connect the UDM Pro's WAN2 to the LB2120 it gets an IP address from the LB2120 and from there, it connects to the internet.
I could not get the WAN2 to connect to the RUT240 no matter what option I tried. I have logged a case with Ubiquiti Support and the guy concluded that the UDM Pro is asking for an IP address but the RUT240 is not giving it one. It receives an IP address when it asks the LB2120 for one, just not the RUT240. So he suggests the problem lies with the RUT240.
So the current combination which I got it to work:
RUT240 (passthrough) - LB2120 (router) - UDM Pro WAN2
I've tried all the different combination of passthrough and bridge for the RUT240 and LB2120 and they don't work - WAN2 does not get an IP address.
But with this combo, WAN2 shows an IP address pretty much as soon as you connect it.
I'm trying to figure out how I can get Teltonika support to fix it.