Pitch it and buy a new router. An old "N150 device" isn't doing you any favors even if you could get moderately secure and operation firmware on it. There are budget options available for under US$20 that are likely to have good support for several years to come if you're not ready or willing to go for the "enthusiast" all-in-one routers or a router + dedicated AP setup.
Very long thread, many opinions and suggestions: