I have a C7 (only C7 nodes) mesh that works - but its only 50-100Mbps on v22.
Mesh is better when carried out on the same chipset or router as a general rule of thumb.
The C7 is not a good match for the Cudy X6 by the looks of it.
Possible workarounds would be to....
create a 2 node Cudy X6 mesh - more likely to work.
on a mixed mesh try rolling back the wireless radio settings to see if it will connect: ie. 40Mhz channel instead of 80Mhz, etc..
If mesh is too much of a headache then go ap/sta instead. Ap/sta almost always works with mixed routers.
Use the Cudy X6 as your main router as it is newer and faster (?) than the C7.
Other things to try...
make sure all devices are using wpad-openssl and dnsmasq-full (I know this is recommended for C7 and probably Cudy X6 too - up to you to test)
make sure the C7 is using non-ct drivers.
retire the C7 to VLAN duty or dumb AP.
I used my old C7 for a VLAN edge router up until recently (replaced with a E8450 to match the new 1Gbps ISP connection). Also if I take a lightning strike hopefully it protects the more expensive equipment downstream. The C7 will do 700-800Mbps on v22 with software offload in my tests with a 1Gbps ISP connection. Still a great router with many possibilities. An N5105 mini PC I tested recently couldn't beat it at straight through routing.
The reason was probably that the mesh was cycling - the number of hops in the path was growing every minute. Also there were 00:00:00...hops on GW and I simply spent too much time trying to troubleshoot it already. There is not much information about the mesh internals to learn what could be the reasons.
To me it seems that mesh (802.11s and wpad-mesh-wolfssl) in OpenWRT is not yet fully working. I ended up with AP/STA in WDS mode.