Why the OTA cancelled the ticket is irrelevant. OP complained that UA cancelled the ticket. That was avoidably misleading and the thread title ought to be changed.
Indeed it makes sense that the TA cancelled the ticket when it came to learn that it would be the subject of a DM which it could not likely pass on to the passenger. But, that does not change the underlying problem. UA cancelled the ticket at the request of a TA acting on OP's behalf.
OP may well have a claim against the TA under local law for the difference between what the TA offered and he must now pay for a ticket. The fact that the TA, on its website, acknowledges what is apparently a routine problem, e.g. under-pricing tickets, may well work in OP's favor.
But, none of this is a UA issue.
Thus, I come back to the real bottom line. What does it cost to book the ticket OP wants? It may be not particularly much more. If that is the case, I would immediately book the new ticket and then deal with the TA afterwards. Or not pursue at all.