You have a few not very good options for overcoming resistance, and none for overcoming immunity.
Your best option is probably the Elemental Adept feat, as it is "permanent".
But it only overcomes resistance, not immunity. This is probably the only really viable option in the rules as written, and it's not great, but it does best fit your character's needs.
I can find no way to overcome immunity.
There is the Elemental Bane spell. But that only covers resistance, requires a save and is concentration. Not a good option.
You can also look to spells abilities and items that grant vulnerability, as this is pretty close to the same as eliminating resistance (but again, it does nothing to immunity). Spells like Hallow and Contagion (flesh rot option) do this. Or Channel Divinity from a Grave Domain Cleric.
Any other ability I found is related to switching out the damage type, ala the Lore Wizard, which is not exactly what you want.