Advice to create a network behind provider's router


I am looking to advice on how I could setup my network so that I can isolate "IoT" devices.
Internet is received from the provider's router which is establishing a network.
I have a Netgear WNDR3700v4 with OpenWRT 21.02 and Devolo DLAN WIFI CPL plugs.

I need to use the CPL plugs to give access to computers and I manage with them as well the WIFI for tablets/phones/etc...

I was thinking about putting the Netgear behind the router, so it will have its own address in
Then I was thinking about creating another network (for example from the Netgear where the CPL would be connected.
Then in OpenWRT, creating another WIFI network isolated from the "home" network of the Netgear+CPL but with access to the WAN which is in the network.

Would that make sense?
If yes, how would I go about in configuring OpenWRT in the Netgear?
I was thinking about connecting the router to the WAN port of the Netgear, but I am a bit lost in what I would need to configure.

Thank you