I do think in some ways, because UX is constantly being pushed back even though it shouldn't be...you do kinda have to learn to work with what you've got. Programming is like that too. There's weird restrictions from legal, stupid restrictions from the platform, some off the wall glitch that you have to work around, etc. Sometimes small UX changes are hard to implement in code and it ends up not being worth doing.

I haven't worked in a startup before. But it's strange to me that a start up would have someone on hand who's work is "not important for the company right now." Aren't startups supposed to run lean? If they won't even -talk- to you about it I feel like your options are limited. If you could actually discuss it, then yeah, maybe you could figure out how to move forward. But 4 months is a long time to not be contributing at all. Though I know that security and UX both suffer from being pushed to the side in favor of new shiny features.

I would:

A.) Have not a full blown meeting, but a 2 minute conversation. Figure out a short example of how not planning UX ahead of time has failed and is hurting the business currently. Then explain "We could prevent this by..." Present problem and your solution.

B.) Use this time to study. This would help with the startup(once they get themselves together) and also your career. Maybe you need more FE dev skills to figure out how to fit the UX in with what they are doing.

C.) Yeah, start putting feelers out. I personally would hate to sit there not contributing. Don't just quit(I assume you need to eat), but there's no harm in just talking to people and seeing what's out there if you really can't make it work at the startup.