Complete this mission before you head to The Isle of Mists. It becomes available after Ugly Baby.

There are three Brothers in Arms quests all in all. The Novigrad ones involve the following characters:

1. Zoltan Chivary: If you've completed A Dangerous Game, he will join you.
2. Vernon Roche and Ves: If you've completed An Eye for an Eye, both will join your army.
3. Sigismund Dijkstra: If you finished Count Reuven's Treasure and stealthily infiltrated the Witch Hunter headquarters and asked Menge about the lost treasure, Dijkstra will lend financial support.
4. Triss Merigold: If you helped her evacuate the mages and completed A Matter of Life and Death and Now or Never, Triss will join. This is not dependent on Geralt pledging his love.

Visit each character (talk to Sigi even if you failed the stealth mission -- he'll just tell you that he won't join), and then move on to Brothers in Arms: Velen.