Mineral rights unrecorded/can't find them

my husband inherited rights to 55 acres of mineral rights in Cass County Texas when his father died in 2018, but I am now assuming they were never recorded, because I cannot find any record of them in Texasfile. I see on there where his father deeded land to another person but in the recorded sale deed he retained mineral rights for his heir.

What do I do about this?

Thank you in advance!!