You are trying to avoid ESD into the semiconductors, so the TVS goes from the antenna input to the antenna ground. In other words, if you have a u.FL connector or similar, it goes across it.
Returning antenna ground to USB ground or earth ground is a different question that is hard to answer since it depends your system design.
Every wire in or out of your box needs ESD protection of some kind.
A plastic enclosure does not really play a part in ESD but I would think you wanted a shielded enclosure (nickel spray on the inside) to avoid EMI. With a conductive interior plastic box, I would try to connect it (via the board standoffs) to earth ground, if possible. If that’s not available, then you can use ferrites/caps/inductors to connect it to what you have or choose to float it.