Maybe a bit late for the thread owner ... but I got my router just a few days back.
YES, it works.
Of course you can connect any other WAN access box to this port, because the login to the ISP is already handled by that box and NOT by the DSR-1000.
So, I user WAN1 for vDSL and WAN2 for LTE, currently working in fallback-mode. As soon as I figured out how, I'll use Traffic-management to use LTE for a dedicated virtual LAN.
For LTE there should be another possibility. You can connect a LTE-stick to the USB port. This is then WAN3. The difference is that in this case WAN2 can be setup as DMZ instead.