The key thing to remember here are that there are thousands of theological details someone can be wrong about. Polygamy being just one of them. Every single individual, mission board, denomination, is wrong about a whole pile of stuff. That includes you and me - we just don't know what we're wrong about.
So, should you not work with a mission board who differs in their view on the celebration of Easter vs Passover? Should you not send a new convert to a church just because their pastor preaches a pre-trib rapture and you think he's wrong? Should you have a really serious special meeting with your mission board about whether hormonal contraception is permissible because someone might get upset if you personally equate it to abortion, and may eventually object to paying to support the 10 kids you think you might end up with as a result sometime in the distant future?
Or should you focus on the Gospel and not let disagreements over such peripheral issues get between you and saving souls?
Just ignore polygamy. Focus on the fundamental task of salvation. Answer questions honestly if and when they arrive, but don't be the one to rock the boat. Just do your job and leave the boat rocking to anyone who decides to challenge you. Build up a solid reputation as a reliable and dedicated worker so that anyone will have to count the cost to the mission before even considering challenging you.