I'm pretty new to OpeWrt (like 1 day).
I'm using a cable modem which is connected to the Wrt Router. I was able to download and install LuCi. To be able to connect with the modem, I setup a static ip with a /22 mask.
I removed all (but one) networks, made sure DHCP is running and created a WiFi Network.
I can connect to the network, but devices seem not to be able to reach the internet... What are the typical mistakes? How can I troubleshoot.
I don't want to run "dumb AP" mode, I'd like to have dhcp, fw, etc on the router and just use the modem as the gateway.
I am not sure what changes you made exactly and why. OpenWrt should work out of the box for a typical use case like yours. Normally all you need to do is to enable the WiFi and set the password, connect the WAN of your router to the modem, and put in the settings for your WAN port (it's a DHCP count by default, so if you are getting PPPoE from your upstream device, you would client need to configure that).
What device do you have? If you have done some changes that you are not sure about, it might just be easier to reset to the defaults and start over.
On cable the modem will only hand out/pass through DHCP requests in a short time window after starting up. So try to power down both your OpenWrt router and the modem. Wait for say 5 minutes just to give your ISP's headend time to properly note that the modem is off (this step is not strictly necessary, but I remember a charter technician tell me to wait a bit). Then power up both the modem and the router, which typically will have a better chance than hooking up a new router to an already up and running cable modem.
Alternatively you could clone the old router's MAC address to the OpenWrt router's WAN interface, in that case hooking up to an already running modem should work as well.
Please note that I last used a DOCSIS network, 2013, so I might be not too precise on details and have no chance to test this before posting my response...
@1) ideally you will set the DOCSIS modem into bridged mode, and simply connect the OpenWrt router via DHCP, or depending on your ISP via ds-lite. Your description sounds like you might still be using the DOCSIS modem in router mode, but even in that case all you should he required to do is configure the OpenWrt router's WAN interface to use DHCP... Static addresses for DOCSIS are possible, but as far as I understand rarely used for non-business plans.
@2) Connect WiFi and switch to your OpenWrt router and you should be good to go. Worst case with double NAT....
But to roll back a bit, a few questions:
A) Is your modem working as plain bridged modem or as router? If you have WiFi devices connected directly to the modem, or a phone, this would probably indicate modes-as-router.
B) Did you try the power-down and -up dance as described above already?
C) What ISP and plan are you using?
I just soft restarted the router and rebooted the modem. That was the router got an IP assigned by the modem and I was able to configure everything else.
Thanks for your assistance
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Glad, that it seemed to have worked out, welcome to the OpenWrt community...