Setting up the CPE for house C in AP Router Client mode alone will by no means isolate the main subnet from the one used in house C. Any device in house C's network still can reach any other device in house A/B's subnet if subnetting is done just on the CPE using AP Router Client mode.

Remember, that there is a default route in the CPE's client subnet pointing to the WAN in AP Router Client mode, which is the house A/B's network, so the main router will route data from any foreign device in house C to any other local device in house A/B's network. Of course it will prevent announcements of multicast services to house C's devices and it also will isolate house C's devices from being accessed by devices in the main house A/B network, but not the other way around!

To ensure that house C will not be able to reach the network on house A/B you have to set up two different networks on the main router which does forward data from both subnets to the Internet, but at the same time can isolate both subnets against accesses from each other. Most ISP routers have the ability to create a separate guest network. Just connect your CPE for the wireless link to house C with such a guest network. Operation mode still can remain AP mode at house A and Client mode at house C in this case.

Mutumba23 wrote

I have setup a guest network on the router now. The router is a Netgear r7800. How do I chose the guest network to be used on the cpe210 in house c?I am using the web portal of Pharos.

Assign an Ethernet port of the Netgear to the guest network and plug in the CPE210 in house A, which feeds the CPE210 in house C. No changes in both CPE's setup are needed.

Mutumba23 wrote

The problem is that the both CPEs on the roof of house A are connected to each other with Poe. So There is just one Ethernet cable from the Netgear router to both the CPEs on house A.Do I need to feed one of the CPEs on house A with another Ethernet cable then?Is There any other option?

You could use VLANs to combine both subnets on one cable. But I don't know wether the Netgear router supports VLANs, you would have to find out. If it doesn't support VLANs, you could use a managed smart switch to separate the VLAN trunk to the CPEs for connection to the Netgear. However, an additional Ethernet cable might be easier to install then setting up VLANs if you have no experience with VLANs.