To play on regular Battle.Net, you'd need to select a different Gateway at the main menu. Hell Unleashed works on the ArimythNetwork gateway, which only has the HU realm.
If you're playing with D2SE, then I think the gateway availability should be set by the mod.
If you're playing without D2SE, and you'd like to play Vanilla you'll need to upgrade to v1.14, so consider making a copy of the entire Diablo2 folder if you think in the future you might want to try D2 mods that aren't compatible with v1.14. To play Vanilla, even if you reinstall, you might need to edit the registry, or use the BattleNet Gateway Editor This link seems to handle either case, let me know if it helped.
https://us.battle.net/forums/en/bnet/topic/15140994487