@Rag, oh sorry I misread your post. Yep you were correct! I blame being sick. Well, partially correct. See in my last paragraph - there are both Teleportation Circles and Teleportation Portals in 5e - the former is only a destination for said spell, but the latter can be used by anyone with the correct passkey. If these "Netherese portals" are the former, then we shouldn't be able to use them.
I agree with @Maximuuus that portal usage is not common; the portals in cities are Teleportation Circles and thus require casting of that spell to use them. So use of them is reserved for Important People that can afford the cost of a level 9 caster - definitely not used by commoners or even Tier 1 (level 1-4) adventurers. 5e Adventurer's League rules suggest that getting a level 5 spell cast costs hundreds of gp.
BUT - there do exist portals around in the world which are usable by anyone with a password or special item. These are relatively common in Adventure Modules; a party will typically find 1 or maybe even 2 through the course of adventuring to level 11+. So there is precedent for portals that don't require casting that Teleportation Circle spell, although I can't find rules governing the creation of these. But again, these are still uncommon even for adventurers; they usually link to a powerful enemy secret base and aren't just out there everywhere in the world.
