How to secure an OMV serwer in connection with Asus RT-AC87U

Hi and welcome.
I am a new user of the forum so at the begining I apologise for any mistakes in placing posts in not right threads.
I have just installed OMV and its up and running very well but I want to secure my data sa much as possible. I have got also a Asus Router RT-AC87U which is connected to Nokia modem from my ISP (Nokia in brigde mode). I have a ddns enabled in my asus router lets assume: myname.asuscomm.com
I have also enabled Lets Encrypt 3.0 on my router.
My serwer spec which is connected to my Asus:

CollerMaster Elite 110A Case
Motherboard AsRock J4205-ITX
2 x 8GB GB Kingston RAM
1 x SSD 60 - OMV
1 x SSD 512 GB - for DATA
PSU picoPSU 160W + 80W adapter

What should I do at the moment to secure my NAS?
Do I have to enable Lets encrypt again from a docker?
I would like to run an OpenVPN - but where? on my Asus router or on my server?
Thank you for any help in advance

Zitat von WastlJ

What do you want to archive?

Who should access the server and where are they located? Within your private network or outside of it? What services do you want to provide?

Hi,
Thank you for a replay. Off course forgot to mentined that important info.
My server is mainly for two things: as a media server in local LAN and for file sharing with my family located inside and outside network.