Subnamespaces are created by adding the namespace into the link.
[[level1:level2]] creates the page level2 in the namespace level1.
What you may want do to is [[level1:level2:start]] which leads to the startpage in subnamespace 2, which is in namespace 1. The same behaviour is created by [[level1:level2:]], caused by the : at the end.
Have you already discovered the page https://www.dokuwiki.org/namespaces
, where the system of namespaces is described?