I had exactly the same issue when I first got my X98 3G.
I tried a few cables/adaptors & contacted my supplier thinking I had a faulty unit.
on their suggestion, I tried cutting back the casing on the micro HDMI plug, so that it entered the socket a fraction further. Suddenly it was working, but not reliably as slight movements could break the connection.
Now I knew, the port worked, I splashed out on a decent HDMI cable instead of the cheaper ones I’d tried before & it’s been working fine ever since, including a second (more portable) cable.