I still find it perplexing why it takes 15 minutes for the problem to occur and why operating the remote prevents it (going by your OP).
Just as an aside, have you gone back to Trustpower and told them the TV they gave you is (may be) faulty? Your reasoning being that the TV it replaced operated perfectly with the same setup. It was essentially DOA when you got it and level 4 restrictions have prevented you getting a replacement. If they swap the set for another and the problem remains it is no loss to you (except for a bit of time). If it removes the problem even better!
Im not sure what video input and output info your Denon can display (either on its front panel or on screen) but you should be able to see what the TV is receiving.