I have a nice short-ish line of sight to my concrete garage, but the doors are facing away from my house, and my zigbee network cant reach inside.
Therefore, is there any zigbee unit or extender with A –a receiver/transmitter end, B – X meters of wire and C – another receiver/trasmitter end + mains power?
You could look at using one of the adapters listed under “hybrid / network” and “network” here, with PoE power and the router (not coordinator) firmware installed.
I solved a similar problem by using zigbee bulbs acting as routers in my outdoor lightning on the house and the garage.
Every single mains-powered Zigbee device is a wireless extender - remember this is a MESH network!
I’d suggest a read of " Network optimization":
Don’t think of Zigbee as point-to-point radio links like mobile phone macrocell towers as mesh networking works best with LOTS of SMALL devices that understand the network better than any human can. 
For Z-Wave, Zigbee, and, Thread, the answer is always more small mains-powered devices (and only very occasionally a firmware update or frequency change).
Mains-powered IKEA kit using Zigbee, is widely available globally such as the plug-in switches and bulbs. The battery kit (e.g. remotes) is less compatible and needs “quirks” to fix odd behaviour (firmware updates fix a lot).
If this helps, 
this post!
Thanks, yes I know they all extend the mesh network. But I want something to bridge a gap where I don’t have any units or mains power, and “go around” a concrete obstacle (I doubt putting a socket extender inside the garage will be enough)
I would think more people would need to do this, or need other forms of extensions to a remote location without “meshing” the entire distance.
Updated my sketch.
If you change the bulb/router to a switch port with PoE (or PoE-injector) on the lowest box you get what I described above 
the router (coordinator with router firmware) would be powered by PoE since you specified the need of having it on the end of X meters of wire. It will then be a part of your mesh as every other router.
Yes, but I want the wire to also carry the zigbee signal physically around the concrete to a router on the other end.
I could do this, as I have seen some people do, but then I understand that the POE-thingy would need to be a coordinator, and create a second network.
No, it would not need to be a coordinator. If you install the router firmware it works like any other routers, extending your mesh. It would mesh with your existing network, not carrying any signals over ethernet, just power.
If you on the otherhand install it as a router and create another network it would communicate back over ethernet, yes.
Yes, in router mode, it would need to receive the Zigbee wirelessly? So then I would still need to bridge the gap.
Correct. I think it would work, my garage is 10-15 meters, two floors and several walls from my coordinator and I have no trouble reaching zigbee devices in it over some outside bulbs. A zigbee device with an external, several meters long antenna does not exist to my knowledge, you would have to make something yourself if possible.
If it does not work you could just install coordinator firmware on it, have it communicate back over ethernet and create a second network in your garage.
Wouldn’t two outdoor smart plugs bridge the gap easily and fairly cheap?
Most likely, or outdoor lighting with zigbee bulbs.
This feels like a Scotty, “You can’ae change the laws of Physics, Captain” moment!
Any mesh router needs to be awake all of the time to listen and re-transmit messages so can’t sleep to save energy like a small mains powered sensor. Zigbee, Z-Wave and Thread are all the same for this reason - routers need mains power.
Worse, even if such a routing device existed with batteries, every Zigbee device connecting to it would likely also have terrible battery life as they’d need to increase their transmit power.
The closest device is a re-flashed Sonoff Z-Bridge. This makes a terrible co-ordinator (complete with a specific warning from HASS developers and a detailed issue report) but with new firmware you can re-use the ZBridge as a Zigbee mesh router and USB power.
I guess you could also modify a mains-powered device to run off a large capacity battery (check the relay coil voltage, remove the AC PSU, add a buck-boost DC-DC converter), but some form of constant energy source is going to be needed.
For me, if you’re going to run low-voltage wires for PoE or USB / DC power, the extra effort to pull in a mains cable and plug in a Zigbee bulb/ socket is negligible. The amount you’ll save in batteries and pain will be worth it.
Depending on the location (I am assuming a country in North Europe based on OPs name), it’s not allowed by law to do this yourself. To have a licensed electrician to do this adds quite a lot of cost.
Given that he has three light bulbs in his garage then there is (most likely) mains already.
Yes, hence my suggestion to use bulbs.
I basically “just” want a mains powered router with two antennas, where one of them is connected by a wire of X meters that I can stick to the roof.
But yes, a POE router somewhere outside will probably do the trick for me. (Might even be possible to power by solar?)
And yes, I have mains in the garage. (But the three bulbs represent my house.)
But let’s for argument say I had a very long stretch that i did NOT want to mesh…
Then that’s not going to work with Zigbee… but WiFi (2x Ubiquiti airMAX PtP can go several km), cheap and nasty 433MHz, even LoRA might be more suitable.
(The irony is there is a new and incompatible version of Zigbee in the works which is NOT mesh, but hub-and-spoke apparently for longer ranges. No, I don’t have any dates.)
