That works often on non openwrt routers, however for openwrt you need to use wan port for the second router.
But when you do there will be another issue, you will have two firewalls and two dhcp servers, depending of your use case and reachability you may choose to let it be as is.
But if you really need to extend lan and reach devices, then you want to search for dumbap type of setup, from what i know this cannot be easily done from the gl ui, i wish they will add support for it in a form of 'bridge' mode, you need to navigate to the advanced settings in luci.
this means you want to add wan eth1 into br-lan, remove wan interface, turn protocol of lan from static to dhcp client or unmanaged, and remove all firewall zones, you could reserve one port with its own network for maintenance.
to add devices or removing ports from bridge br-lan you navigate to:
Luci -> network -> interfaces -> devices (tab) click on edit, you can add eth1, and uncheck lan5 or lan4 (the last port), click save but not save and apply.
If you click back on tab interfaces next where devices are, you delete wan.
Then edit lan and change protocol to dhcp, on the firewall tab you select none.
again don't apply yet only save, you want a safety maintenance network.
now create a new network interface, call it maintenance and choose lan4 or lan5 the one you previously deleted from br-lan.
Select static protocol, under address you can type:
192.168.32.1
subnet: 255.255.255.0
gateway: leave empty
Navigate to firewall tab create a new zone maintenance.
On dhcp you click on create.
Now navigatie to luci -> network -> firewall and navigate to the maintenance zone, change input to accept.
Now ensure you are connected to maintenance port and click save and apply.
Note that alot of logic will break within the gl ui, but yea... this is a very legitimate setup not advanced perse but in the time present the ui was not made with these intentions back then, hopefully in the future it will be a function on its own