a 401k and a IRA are totally separate accounts. a 401k has an $18k yearly limit and an IRA has a $5500 yearly limit.
A 401k is special because within it you can have a traditional portion and a roth portion, if you so choose. Sounds like you have both these two sections within your 401k "bucket". The $18k limit applies to the entire 401k bucket, so both your $14k and $3.5k apply to the $18k limit. Yes, the portion you allocated as roth will miss out on the tax deferred growth, but roth money is still tax advantaged, you just paid the tax now instead of later.
You can open a separate roth IRA (or traditional IRA if you meet the requirements) and contribute $5500.