networking

I had that exact same modem. I don't see any issues with not having a firewall unless you're trying to actively block outgoing connections to some domains. Incoming connections are already going nowhere by default because of the NAT: the modem gets the public IP, and unless there's a forwarding rule to send it to a local device, it acts as a defacto block all incoming firewall (unless you enable DMZ of course, or the passthrough option to use your own router).

I think it's a fairly decent modem/small router overall. Unlike some other brands, it doesn't bother with implementing a million features almost nobody uses that could cause instability. It routes, it forwards, it WiFi's, that's what 99% of the people need and want. Unlike other "try to do too much" routers I've had, this one's been rock solid and delivered the advertised speeds.

Basically, it gives you Internet without interfering with it, and you're free to add whatever you need yourself if you so desire to. But for most cases, it's fine.

Or is the router doing some packet filtering or something along those lines without me knowing

A router does NAT and that means no incoming connections being made from the internet to your public IP will ever reach the machines inside your network. They'll simply be dropped / ignored. Only if a machine initiates a connection then a response from some server on the internet will be forwarded to it. Because of this NAT is in itself a very solid firewall that protect your computers against threats coming from the internet.

https://whatismyipaddress.com/nat