Apples don’t reproduce true from seed.
What you can do is find some rootstock and/or someone who is familiar with grafting and graft some sprigs of your tree on to some appropriate rootstock.
What you may have is the rootstock itself. Since you mentioned you let it grow from the root.
IF so, then just root some cuttings next spring when it starts to bud out. Lots of places on the net that give advice/tips on that. My only added tip would be to pick a root/transplant day from the farmers almanac calendar.
Non-hybrid apples do great from seed. This is a non-graft, so that doesn't apply.
/johnny