Messed up, could use some help

So the other day I was installing openwrt on a new netgear router I had purchased, connected to my laptop by ethernet cable. When I was done, I took the router to the other room to connect it to my cable modem...and I guess I had left the cable plugged into the router and without thinking just plugged the other end of that cable into my modem, so I basically connected the router to the modem through one of the LAN ports instead of the WAN port. I know a bit about networking but not really the nitty gritty details, but I had an inkling that this was a bad thing to do and that I had messed up. I searched for info and got conflicting opinions on the topic, ranging from "your network was pretty much exposed to the internet" to "due to NAT errors your devices would have been unreachable anyway", so I couldn't really make heads or tails of it. I will say that the modem was plugged into the LAN port of the router for maybe around 5 minutes, and while it was, my router IP was unreachable from my laptop over wifi. The router had wireless network active and there was nothing else plugged into any of the other physical ports.

Can anyone please give me some input as to what happened here? I'd really appreciate it. Was my network really exposed to the internet without a firewall? Are there any measures I should take? Maybe a silly question but could my cable modem be compromised somehow after this? Appreciate any info on the subject, thanks!

Your cable modem will only allow one connection.

The router wouldn't even be able to establish a proper connection to the modem while it was connected the way you described.

The router would have been partly acting as a switch.

Of the devices you connected one of them may have connected to the modem and hence the internet.

The device that managed to connect to the internet may have been at risk depending on the firewall settings.

You could reinstall openwrt on your router without keeping settings to be safe

Do a virus scan on your PC that connected to the internet during your mishap.

Power cycle all your non PC devices.

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Hi mbo2o, thank you for the reply. I realized some parts of my post were too vague. Basically, the ethernet cable going from cable modem was plugged into one of the openwrt router's LAN ports, I had the wireless activated, and my laptop was connected to the wireless network during this. I was unable to open a web page or even access my router's config page through the wifi. I don't recall if I had messed with the firewall settings but i believe the default LAN rules were in place.

Sorry but I don't entirely understand your post. Are you saying that the laptop was not even connected to the internet because the router didn't have a proper connection with the modem? How else the laptop "may have been connected to the modem and hence the internet" as you mentioned later? If you perhaps mean a device connected to one of the other LAN ports then no, the rest were all empty. The WAN port was also empty.

If you couldn't connect to internet with that configuration then most likely nothing bad happened.

You just need to do a check on any device you know was successfully connected, probably none.

Stop worrying,