You say that as if (normal) domains are expensive. You’re gonna be paying a lot more for electricity for your NAS than a domain. If you don’t need anything recognizable which you just want to use for yourself, you can even get a 1.111B class domain (000000.xyz - 999999999.xyz) which are just $1 per year. It’s a much better option than a dyndns service because you can actually do whatever you need to with the domain.