Unfortunately, you can’t return to Asgard in God of War Ragnarok. After the end of the game, it appears the realm is destroyed. The only thing you find of it is smoldering pieces thrown into other realms. It’s even removed from the Mystic Gateways and isn’t a selectable location.
Can you unlock Asgard in the God of War Ragnarok post-game?
You can’t unlock travel to Asgard in the God of War Ragnarok post-game because it’s likely been destroyed. Ragnarok goes berserk during the final battle and begins carving through the realm with Surtyr’s Sword. Kratos and the party narrowly escape death by escaping through a realm tear, and it’s almost certain Asgard’s destruction was complete shortly afterward.
Fortunately, there aren’t any missable collectibles in Asgard. So completionists don’t have to worry about being locked out of a 100% run.
Lore-wise, though, we don’t know what happens in Asgard after escaping during the ending of God of War Ragnarok. The prophecy stated that Ragnarok would completely destroy Asgard, but the fact Kratos survived shows that it wasn’t entirely accurate. So it’s possible that Ragnarok’s rampage was stopped, and at least some of the realm remains. However, we don’t see anything in the post-game that indicates that’s the case.