What do you mean, your ex-husband “filed you.” What was the date of your divorce? Have you filed your own tax return for 2020?
File your own return as single (or head of household if you have a qualifying dependent). You will have to file by mail, if he e-filed first and blocked you. You can claim the rebate on your return if you did not receive a stimulus payment over the summer. When the IRS gets the two returns, they will first pay your claimed refund, then they will start to investigate. If your divorce was final in November, it will be easy for you to prove you are right and he is wrong. But don't send proof now, just sign and mail your return and wait.
If you received a stimulus payment over the summer in your name, but your ex kept the money, the IRS will not pay a rebate now even if you claim it. That is a financial issue that should have been addressed as part of your divorce.