How to route to a client (modem)?

Installing and Using OpenWrt Network and Wireless Configuration
1

I have a unmanaged PoE switch 8+2 but it has an option to isolate all clients but a WAN1 and WAN2 ports.

If I connect an LTE modem to PoE port 8 and OpenWRT router to WAN1 how do I configure the OpenWRT router to route all the traffic to the modem?
I would like to avoid permitting other network devices accessing the internet aka the modem.

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Switch is SF10P-HM and although isolation mode has name VLAN I did not notice any VLAN tagging in the documentation that I could take advantage of setting up a WAN interface.

2

Some call switch partitions VLAN-s, i.e groups of ports. You need to connect your router with multiple cables to decode them.