Yes, ĉi-monato is wrong.
"Ĉi monato", without the hyphen, could be shorthand for "ĉi tiu monato" which means "this month", but it's a noun phrase, not an adverbial.
So the sentence would be ungrammatical because it would seem to have two subjects.
Using ĉi as a separate word, without a word from the ti- group, is a bit unusual and I would recommend avoiding it.
So, stick with ĉi + ti-, or ĉi- (with hyphen) + adverbs.