Riyan is indeed nowadays in Yemen. In my day (as an Argosy captain with 105 Squadron based at Khormaksar in Aden) Riyan was in what we called the East Aden Protectorate.
If you Google Earth "Riyan Airport" you will still be able to see the old strip to the west of the "new" airport. Riyan was the airfield for Mukalla which at that time was the HQ of the Hadraumi Bedou Legion (a British officered camel corps).
Every day, one of our Argosys set out from Aden on the "RSM" (Riyan, Salalah and Masirah) supply run. The Riyan runway was kept in tip top condition and was as near as you can get to the surface of a billiard table.
I remember the ground equipment being sold off just before we pulled out of Aden in 1967. The immaculate fire engine was flogged off to a local (who intended to use it as a water carrier) for around £100!