Welcome to the Kitz forum.
Yes, I have heard of that scenario but not specifically with TalkTalk. As you appreciate, establishing an Internet broadband service is a multi-stage process. It appears that the Openreach technicians who have been tasked to attend to your problem have identified that is not in their domain of the physical infrastructure. From the evidence presented, it does appear to be centred on failure to establish a connection to the bRAS.
As is the normal reaction, from basic CPs/ISPs, any problem must be down to somebody else's equipment. A problem is reported to a basic CP/ISP and they, in turn, will insist that the problem is with and of the end-user's making. After a lot of script-reading, by the basic CP/ISP, and hoop jumping, by the end-user, the basic CP/ISP then decides that the problem is within Openreach's domain.
There does not seem to be any easy way of surmounting the barrier installed by TalkTalk. Quite simply, it might be best to go along with their scripted nonsense and let them send you another modem/router. The only other possible way would be to insist that they allow you to terminate the contract with them, at no cost to yourself, as they are unable to provide the service for which you are paying.