Sure it is possible, although it makes not much sense. First of all, real-live distance is limited by the main router. Second, repeater mode often introduces more problems - like hidden node problems - than it is useful. Client mode is preferable. Third, emission patterns of the CPE's antenna are directional (60° for CPE210, 45° for CPE510), not omnidirectional. CPEs are meant for a outdoor PtP or PtMP setup in WISP configurations and this is where they perform best.

Again: it is an outdoor device. Not useful in such an indoor setup.

w20grippy wrote

What would be the possible solution to get the signal from main router via wireless. ?

I would recommend to place the main router in the center of the area to be supplied, not in a corner.

If that doesn't help, first choice would be to place another AP (not router) in unit 3 and use a wired connection to the WR841.

If wired connection is not possible, next try would be a PowerLAN/DLAN connection.

If PowerLAN/DLAN isn't possible, you could use an indoor repeater, but this might be somewhat unstable due to the possible "hidden node problem" (see https://en.wikipedia.org/wiki/Hidden_node_problem), so it is usually the last alternative. Wether the HNP might be a problem depends on wether your WR841 can see client devices in unit 3 at least (although a connection or a data transfer is not possible) or wether the WR841 can not see the clients at all. If the latter, using a repeater doesn't help that much.

w20grippy wrote

What is your stance on that?

Same. You can't align a directional radio beam of the CPE to a "wireless zone" inside a perimeter outside the house, but you would need to align the CPE to the antennas of the WR841, so that any wall in between the direct line from CPE antenna to WR841 antenna will contribute in degradation of the signal. The directional characteristic of the CPE's antenna generally extends the radio link's distance compared to omni antennas, but you would need to have a free line of sight and two equally strong antennas on both sides for a good connection.

Next point is the WR841, which also has to send a WiFi signal to the CPE. If it can't feed a repeater, it probably won't feed a CPE any better. Although the CPE's receiving sensitivity might improve the situation somewhat, it is not granted that the WR841 could send a signal strong enough to reach the CPE through walls.

Thus, there are too many "mights" and "coulds" to give a clear recommendation for such a setup. Maybe it works, maybe it only works somewhat better than with a standard repeater, but changes are that it won't work at all due to a weak signal from the WR841 back to the CPE.

If you can mount the 1st CPE outside the house (Unit 3 is an own free-standing house? And there is an unobstructed directional free line of sight possible? Important information you didn't provide in your first post!), then place a 2nd CPE on the building's edge or roof wired to the WR841 as suggested by Johnsontp in post #7. Align the antennas of both CPE's exactly to each other and it will work for sure. This is something at least Johnsontp and I could recommend. :)

Absolutely correct, what Victor did wrote.

No matter which type of WiFi gear would be used as the remote device for the WR841N, the radio link at best will achieve the quality of a WR841N itself only, because the WR841N's device characteristics is what counts if it takes the role of the sender. The same is true for any client devices such as SmartPhones, tablets, laptops etc. used to connect to the WR841N - the radio link's quality will go down to that level, what the weakest device of the particular combination of the connected devices can deliver.