Post your config
/export hide-sensitive file=anynameyouwish so I can see the rest of the config and not just snippets.
Also a network diagram to show the relationship of the two WG routers connected by internet, and the associated wireguard devices / subnets desired on the client side.
I am assuming you mean WG to WG where one is server and one is client??
It actually turned out to be quite simple once I got it, here are the relevant parts of the client config:
# jun/19/2021 23:09:37 by RouterOS 7.1beta6
# software id = NMKX-MZTE
#
# model = 850Gx2
/interface wireguard add name=wireguard-client
/interface wireguard peers
add allowed-address=192.168.100.2/24,N.M.0.0/16 endpoint-address=<WG server IP> endpoint-port=12345 interface=wireguard-client public-key="Server Public Key="
/ip address add address=192.168.100.2/24 interface=wireguard-client network=192.168.100.0
/ip dhcp-client add disabled=no interface=ether1
/ip route add dst-address=N.M.0.0/16 gateway=192.168.100.1
It's a test router, connected to the Internet via NAT, there are is nothing else of relevance. I removed the public IPs, since I do not want to see the Internet test their setups against them.
The WG server s pretty much standard as documented at various places, it listens at <WG server IP>:12345. 192.168.100.0/24 is the subnet for the tunnel and all end points, the server endpoint 192.168.100.1 is NATing WG traffic into the target networks.
As a result, N.M.0.0/16 is reachable from the client router, I guess this should be enough for others to successfully configure RouterOS as a WG client.
Overall, I'd like to remark that the WG documentation (at least the parts I found) is hard to understand and not very precise, so it's tough to get things working. It's also hard to debug a WG setup, since there is no direct indication if a peer successfully connects as with other VPNs. You can debug it to some extend by torch-ing the WG interface and enabling logs of corresponding fw rules firing, but it's not fun. More logging is badly needed. Besides this, speed is terrific, but deployment does not scale beyond toy scenarios with the implementation provided so far. So, I conclude with Dante Alighieri:
Lasciate ogni speranza, voi ch'entrate :-).