I'd say that if you tried with your USB to serial adapter and with a computer that had a built-in serial adapter then the issue may be your URC cable. That is the go between remote and computers that is identical in all scenarios. I agree that it seems unlikely that both remotes are bad.

If you decide to look for a new cable be aware that all that look like what you have do not necessarily work. The pinout connections are not all the same for cables such as these. Just be sure it is guaranteed to work with the URC MX700. There are directions on the Internet (and maybe here on Remote Central) showing how to make a cable.