Rocketchat is a chat system which is mainly suited for private chat with a few family and friends. It has some integration capability with other systems, but isn't federated as XMPP or Matrix are. If you need high security then XMPP with Conversations is probably still the best option.

This system is only available for X86 architecture, so won't install on ARM boards but probably will on an old laptop.

ssh into the system with:

ssh myusername@mydomain.com -p 2222

Select Administrator controls then Add/Remove Apps then rocketchat. Enter your domain name and freedns code if you're using freedns.

Navigate to your rocketchat domain and register an account. The first registration becomes the administrator. It's a good idea within the Rocketchat administration settings under Accounts to select Registration and Manually Approve New Users, then save. This will prevent millions of random internet users from creating accounts on your server.