A Routed Client (Masquerade) is what you want.
In the default configuration, OpenWrt routes and masquerades LAN users to a wired WAN connection such as a cable modem. To make the WAN link wireless, it is only necessary to detach the Ethernet port from the existing WAN network and attach a wifi client instead.
This page seems relevant: https://openwrt.org/docs/guide-user/network/routedclient Follow the three steps in the first section under Masquerade. This is the most appropriate configuration when you don't want to interact with the other network other than use it for Internet access. The other network will be on the WAN or untrusted side of the firewall. The default firewall will work for this.
I recommend manually configuring the wifi client rather than using the scan and join button. That wizard creates a new network 'wwan' which is not necessary. The existing wan can be used.