Best ways to secure the internet modem?

As far as I know (correct me if I'm wrong), you cannot do much of anything to your modem specifically. You can however do a lot to your router and any other network devices.

For example, if you have an advanced enough router or a switch you can set up VLANs for different groups of devices on your network to provide isolation. You can also set a custom DNS on your router or even a VPN if you'd like to prevent your ISP from seeing your internet traffic. Or a firewall (either hardware or software).

It is also generally a good practice to keep ports that you do not need closed. From my understanding, open ports are used by attackers to get into your network using various exploits. You can think of open ports as holes in your network.

I recommend searching NetworkChuck on YouTube (using your favorite FOSS YouTube player of course). His focus is network cybersecurity and Linux, and he does a really good job of putting everything in a format that is very easy to understand, and provides step-by-step tutorials on things as well.

I hope this is what you were looking for, if you have any other questions I can try to help. I studied Cisco networking for 2 years but to be honest I forgot most of it haha

All that u/candroid_man and disable UPnP.

UPnP is a simple why for applications open ports on your router when needed, but also a security risk. Malicious code can take advantage of UPnP and open ports for remote access, or hacker can look for open ports to send exploits.

https://www.howtogeek.com/122487/htg-explains-is-upnp-a-security-risk/

Also disable WPS or enable it only when needed, WPS is a quick way to connect devices to your WiFi without the need of enter the password, some devices like printers or WiFi extenders only have the option to connect with WPS. But WPS can be easily exploited with bruteforce.

https://www.howtogeek.com/176124/wi-fi-protected-setup-wps-is-insecure-heres-why-you-should-disable-it/

PS: I use to exploit WPS networks with termux (Android) when i need a network, and it can be done in a few seconds.

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If not installing security updates on the modem/router automatically, manually do (check for any available and install) so on a very regularly basis (at least once a week), if not possible to configure it to do automatically.