The free servers work, but you wait in queue to start up the server, sometimes 5 minutes, sometimes up to half an hour. Then you have a very short window to log in, and if you don't, the server shuts down and you must start again. Afterwards, other players can join, and you can play, although server draw and simulation distance is hard-limited to 6 chunks (awfully short) and if there are many other players waiting in queue, after about an hour you'll be kicked out, the server forcefully shut down (if there's no heavy demand you can play indefinitely). Never mind if you stand idle for a while you're also kicked, and if nobody's on-line, the server is shut down, so forget about any afk farms running in the background, and even a bathroom break may cost you your session. The chunk generation is abysmally slow - forget about meaningful exploration on Elytra; in general the performance is lousy - but otherwise, they work.
Personally, I'd recommend shelling out some money, even $3 / 3 months for the cheapest hosting on Pebblehost - or just setting up the server on your own PC, the same you use for playing. Aternos works, and is free, but it's NOT a good experience.