mkdir -p /mnt/cdrom

mount -o loop -t iso9660 /root/rhel-server-7.6-x86_64-dvd.iso /mnt/cdrom

vim /etc/yum.repo.d/yum-local.repo
[yum-local]
name=iso
baseurl=file:///mnt/cdrom
enabled=1
gpgcheck=0

echo '/root/rhel-server-7.6-x86_64-dvd.iso /media/cdrom iso9660 defaults,ro,loop 0 0'   >> /etc/fstab

yum clean all
yum makecache

Q.E.D.