Cannot text to one person only

My husband and I both have older version Samsung phones (J3 & 4). We have had these phones for over two years and no problems. Starting this week, I cannot text to my husband and we are both on the same plan.  He can test me, but I cannot reply.  I get a message that says it failed.  I can send a photo, but not a text message.  We have tried everything.  Restarting our phones, resetting our network settings, powering down,  deleting the contact, re-adding it and restarting again.  We both checked that we are not blocked on the other's phone. We are not.  Nothing is working!  This is very frustrating and could be a problem if there is an emergency.  Any suggestions?  Thank you in advance.