Selling unregistered shares with restrictive legend

Dear person,

Few months ago my previous workplace company went IPO. I purchased shares from the company in unregistered, private transactions prior to the IPO, via the exercise of stock options. Such shares have not been registered with the U.S. Securities and Exchange Commission, and they bear a restrictive legend that requires removal before the shares can be traded. We were asked to sign a lock up agreement for a period of 10 months which I declined, and I'd like to be able to sell my shares any time soon. My case is likely rather unique among the shareholders. The external brokerage company is supposed to instruct me re the process, and timing (i.e. when those legends can be removed). They will require a legal opinion to remove such legend.

My question is whether they are allowed to legally decline removal of such legend. Are they allowed to decline the process for any other reason? Basically what are my rights to sell my shares during the company lock up period for which I didn't sign? Also do I have any rights regarding the timing. I've lost already an opportunity to sell my shares at higher price before recent stock market decline because of very slow paper work and email exchanges.

Please let me know if you think there's a more appropriate place to ask these questions.

Thank you for your time.