Ymir is one of the best choices to play support in smite 2 (for me, of course). He has impressive CC, explosive damage and amazing battlefield control.
Passive (Frostbite): all of his skills that hit an enemy combine Frostbite. It's basically a debuff that increases the damage of Ymir's basic attacks and reduces the damage of enemies hit. I always like to follow the use of the Skill with a basic attack, to increase the damage.
Skill 1 (Ice Wall): an ice wall is summoned that blocks movement and displaces enemies when cast and can be destroyed when you reactivate the Skill. If you use it on Ymir's feet, he is pushed forward and it works almost like a dash. You can use it either to block the passage and protect yourself or to trap enemies, just make sure you position it well according to your purpose.
Skill 2 (Glacial Strike): Ymir creates a line of ice spikes that deal magic damage and slow down enemies. I like to use it to clear waves of minions and even freeze them.
Skill 3 (Frost Breath): Ymir releases an icy breath that deals damage and stuns enemies. Excellent for starting fights and stopping that enemy who is your priority target. Preventing escapes is also a great application. The combination of this Skill with Ice Wall is excellent, as you can trap stunned enemies.
Ultimate (Shards of Ice): Ymir freezes the air around him and slows down enemies while channeling. After channeling, it explodes and deals mass damage. Very useful for dealing area damage in team fights and securing kills. For this I always try to position myself in the center of the fight, to ensure maximum benefit.
The build may vary a little depending on your role, but they all prioritize reducing cooldowns, defenses and magical power, always thinking about maximizing your CC and your survival on the field. Items such as Heroism, Blink Rune, Prophetic Cloak, Genji's Guard, Talisman of Purification and Circe's Hexstone are very useful for the build.
Always know how to position yourself, using Ice Wall with the right positioning will help you a lot. As support, try to control the route, place plenty of sentries and stay close to your rearguard so you can always protect when necessary. Ymir is an excellent choice, perhaps with a slight weakness in his mobility but nothing that hinders him too much, after all balance is everything.