I am trying to play Undead Legacy version Alpha 20.7 which is the highest the mod is updated to, while the server host states 21.2 is the running version of the server, Does that mean with this server provider i cannot play Undead Legacy if they arent to 21.2 version?

Correct. This server provider, like many for 7 Days to Die, only support the versions of the game that are officially supported by The Fun Pimps. In this case, that is two versions of the game:
Stable, and Experimental.

At the time of writing, that would be only one version:
Alpha 21.2 (build 30) as the stable version

There is no current experimental branch, as the latest experimental was finalized as Stable on November 20th, and Alpha 22 dev diary updates starting shortly after.
As of December 23rd, 2023, there is no Alpha 22 (build x) for experimental, as it's an internal build.

So if your mod author hasn't updated their mod for Alpha 21, then it's likely the mod was abandoned, and as the game is presumably nearing Beta / Release status in 2024, it's likely it own't be useable at all as the Alpha versions will be removed when it goes "Gold"