You could either use two CPEs connected together by wire on top of the hill as a relay or consider use of WBS210 with an omnidirectional antenna TL-ANT2410MO. However, this antenna is not (yet) available in all countries, I'm still waiting for it to appear in Europe.

But note that repeater mode 1) cuts the total bandwidth by half and 2) might introduce the Hidden Node Problem (see Wikipedia for an explanation). So, two CPEs would probably be better.

Drdan wrote

I was hoping I could connect 2 cpe210s back to back. Do you know of any instruction to do that?

Connecting back to back? That's not possible. They use directional antennas radiating the signal forward in one (physical!) direction only.

Mounting them back to back and connecting them by a cable so that they wirelessly connect in opposite directions with two remote counterparts is indeed possible, yes. Such a setup would be a "classic" relay station then.

The question then is what are the best modes for each of these?

I think this is most obvious but it doesn't seem to want to be bidirectional...

Did you mean symmetric? Any wireless link always supports bi-directional data flow independent of the mode its devices on both ends are working in.

L - AP mode
LR - Client mode
ER - AP mode
E - Client mode

Yes, correct modes. It's not important which one is working as AP and which one is working as client, except if you use a WBS210 with omnidirectional antennas, then the WBS210 would need to work as an AP in order to let more than one CPE client connect to it. With WBS the setup would be:

L - client mode (CPE)
LR/ER - AP mode (WBS with TL-ANT2410MO)
E - client mode (CPE)

I am very unsure of both the relay cpe210s mode. It seems there are other things to consider.

"Relay" just means that it is a station relaying the signal, but it does not describe any mode the relay works in. It's just kind of an extension of the coverage area, in your case an extension of the range of free LoS along the hill up on one side and down on the other side. While in wire-based Ethernet terminology such an extender would just be a signal amplifier often called a "(signal) repeater", in wireless networking terminology it is called a "relay" due to the nature of half-duplex "relaying" (i.e. first receiving from one side, then sending at the other side). Of course, wireless repeaters are relays in this sense, too.

I do not recommend "repeater" and "bridge" modes for the CPEs. They are difficult to set up correctly for a stable link if you want to avoid the Hidden Node Problem, not to mention the half cut in bandwidth for each repeater in-between.

You can avoid the HNP completely by using four CPEs or by using two CPEs and one WBS with omnidirectional antennas.

You are welcome, Drdan. As for update to R2D3 will ask support line at next occasion. ;)