If it answers the question Where?, then it's an adverb.
Q: Where does this router have a range of 100 meters?
If it answers the question What?, then it's a noun.
Q: A 100 what?
A: A 100 meters. ('100' is more than one, and it modifies 'meters', so 'meters' must take -s)
Adverbs cannot modify nouns.
indoor use (adjective + noun) e.g., It has many indoor uses.
*indoors use (adverb + noun)