Can undropped testicle cause any problem with fertility?

Depends 1 or both. Somewhat & hopefully only slightly reduced fertility with 1 testicle up after puberty & other fully descended. Severe to complete infertility if both testes remain undescended. Fertility should be close to normal if single undescended testis was corrected in very early childhood. Fertility is always reduced if both testes were undescended, worse when they were higher &/or later they were repaired.