"Fixie" means NO free-wheel. If the bike is moving, you MUST be pedaling.
"Single-Speed" means that you have only one gear, but have a free-wheel and you can coast.
====================================================
Try placing the chain on the desired cog, pull HARD and tie-off very tight.
Or use the -- oh, what the heck is the correct name? -- hi / lo set-screws to force the derailluer to stay in the chosen middling cog.
The latter worked for me in February this year for a few rides until I had the time and funds to replace the rear cable.
I was riding a three-speed for a couple weeks: 50/39/30 in front -- 15 in the back.