You can e.g. use WHMCS together with the free WHMCS ISPConfig plugin: https://www.howtoforge.com/community/threads/new-ispconfig-module-for-whmcs.67824/
For people from The Netherlands (and I believe it works for Belgium too) you can use hostfact (https://www.hostfact.nl/) But this is also a paid solution.
If I remember correctly, there is also a WordPress Plugin and a Joomla Plugin for ISPConfig, but I don't know which features they offer and if they are free or paid.
