OpenWrt WAN port settings

After installing OpenWrt on my Linksys MR8300, the uradzenie does not connect to the network ( opertator cable modem).
On the Linksys firmware, it immediately downloads the address and works without problems ( by default, the WAN address is taken from DHCP).
After installing OpenWrt, is it necessary to set the WAN port in some additional way so that it downloads the address from the modem ?

no, but the WAN and LAN subnets might overlap, and they mustn't.

where to check/change it ?

Wan port IP/subnet is displayed on the status screen.

You can also check if the WAN MAC in OpenWRT, matches the bottom sticker of the device.

Cable systems often "lock to" the router's MAC and refuse to serve the new MAC when you replace the router on the same modem. To clear this block, it usually works to unplug the cable modem entirely for 30 minutes, then start it up again connected to the new router.

Or you can override the new router's MAC to be the same as the old router (not the modem). In OpenWrt this is under the Advanced Setting tab on the Interface editing page.

I read it as it's the same device running stock or openwrt ... ?
OP might however have connected something else to the modem inbetween ....

That can still cause the WAN port's factory MAC to change by one or two in the last digit, as the two OS may use a different scheme to assign factory MACs to additional interfaces.

After switching from OpenWrt to Linksys firmware ( from partition to partition), everything works without problem. So I assume the problem is in the OpenWrt port settings.

that's why you were asked to check the WAN MAC, and the WAN/LAN subnets.

MAC is the same as LAN

identical WAN and LAN MAC probably won't work....

How to change MAC on LAN interface ?

network -> interfaces -> devices (tab) -> (br-lan) configure -> general device options

Unfortunately, changing the MAC to LAN interface did not solve the problem. WAN still does not work

rebooted too ?

then unplug the WAN cable, and reconnect it, post dmesg.

unfortunately it still doesn't work, could someone take a screen shot of their setup ?

There's nothing wrong with the default settings, and you still haven't answered if the WAN MAC matches the sticker, + dmesg.

Do what you've been asked to.