POTD is not a great place to gain xp for a tank. Not only that no tank stance or cooldowns are required so it doesnt really teach you your job. Not only that, enemies die so fast that you cant even get a full rotation in.

It can perhaps teach you how to hold aggro for under 10s but that is about it.

Best bet for tanking is to run dungeons where 1. You help the leveling queue. and 2 you learn important things to look for and mechanics.

POTD is great for leveling dps and basic gameplay mechanics that you learn from the story anyway, but its not a good tool to use to actually learn your class.