To get others to connect to your BTC node, you must enable port forwarding in your router to the apollo node.

Example:
Inbound rule:
Allow ANY SOURCE IP:Protocol:TCP:ANY SOURCE PORT -> APOLLO INTRANET NODE-IP:Protocol:TCP:PORT:8333

I highly advise to configure suitable internal and external firewall rules, accordingly, and place the Apollo node into a VLAN network which is separate from your normal untagged intranet LAN.

Example:
internal firewall: full isolation of apollo server from intranet network. Except access for specific admin device (allow access to web-if (port 80) and SSH (port 22)).
external firewall: allow access from any external port to apollo server tcp port 8333 only.

As Ubuntu is a very save OS, I have no real doubt to get hacked.
However, you could consider additionally using TOR via flag in settings, to increase security.

You can try out, if your node is accesable using this:
https://bitnodes.io/#join-the-network