You can activate Frodo, Sauron's Bane's abilities regardless of what creature types it is. Each of those abilities checks Frodo's creature types when that ability resolves. If Frodo, Sauron's Bane isn't the appropriate creature type at the time, that ability does nothing.
The Ring can tempt you even if you don't control a creature. In this case, abilities that trigger "whenever the Ring tempts you" will still trigger.
Frodo, Sauron's Bane's first ability checks whether it's a Citizen, and its second ability checks whether it's a Scout. It doesn't matter how it became the appropriate creature type.
Magic: The Gathering · ™ & © Wizards of the Coast · Illus. Dmitry Burmak
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Some spells and abilities that cause the Ring to tempt you may require targets. If each target chosen is an illegal target as that spell or ability tries to resolve, it won't resolve. The Ring won't tempt you.
If the creature you choose as your Ring-bearer was already your Ring-bearer, that still counts as choosing that creature as your Ring-bearer for the purpose of abilities that trigger "whenever you choose a creature as your Ring-bearer" or abilities that care about which creature was chosen as your Ring-bearer.
Each time the Ring tempts you, you must choose a creature if you control one.
As the Ring tempts you, you get an emblem named The Ring if you don't have one. Then your emblem gains its next ability and you choose a creature you control to become (or remain) your Ring-bearer.
Each player can have only one emblem named The Ring and only one Ring-bearer at a time.
The Ring gains its abilities in order from top to bottom. Once it gains an ability, it has that ability for the rest of the game.
None of these abilities have a duration. If one of them resolves, it will remain in effect until the game ends, Frodo, Sauron's Bane leaves the battlefield, or some subsequent effect changes its characteristics, whichever comes first.
