This may sound easy for me to say, but none the less it is the truth. Adultery in any form is not the answer. Using herbal and medical remedies are reasonable methods to do all you can physically, but ultimately God is the one who opens and closes the womb. Do you want to try to force God’s hand? I can say with confidence that trying to make something happen that may not be God’s will, will bring painful outcomes. It’s possible the pain may not be felt by you or your husband even, it might be felt by your resulting children. My advice, and I believe it is biblically sound, is to use what natural/medical remedies seem reasonable to help you and your husband and then trust God to do His will in your lives.
I will be praying that God will give you and your husband children together, if it be His will. May our heavenly Father richly bless you both.