Interesting way to do it, but makes perfect sense also. Here in Germany, we have to plug in these two failure-prone units, old phone line in one of these, computer or router in the other. Only advantage you don't have to change anything in your old phone installations, as the signal is filtered before it is fed into the house's actual phone lines.
Still, that makes a wiring problem unlikely, as phones are working. Possible that the router has a problem where the DSL input is, while the acutal network functions are working. But unlikely. Let us know what the technician will find, I'm curious
.