desec does offer one free subdomain, and you can use as many nested subdomains as you want for your services. do note you'll need a wildcard cert for each subdomain level: *.sub.dedyn.io and *.app.sub.dedyn.io if a service needs it's own subdomains for apis and whatnot.

edit: also a note for any fellow noobs like me it's deDYN.io not deSEC.io on your account/subdomain. it took me an embarrassingly long time to realize my mistake trying to sign up with every subdomain i tried saying it exists already.