Error code 018-0513

[wispy quiver]

Me and my brother are trying to play online together and it gives us this error when we try joining each other

[pure valve]

018-0513 (3DS/2DS)

Support code detected, press to expand information

[burnt hornet]

This could be one of two issues
A. A bad NAT type, since you're still going through servers, NAT type compatibility still applies
B. The game has issues with joining a user who is on the same network as you, such is the case with the Monster Hunter games

To rule out NAT type incompatibility, could you perform a connection test in the System Settings of both consoles and provide the support codes which will be displayed in the bottom-right corner of the top screen

[wispy quiver]

They are both 120-00

[burnt hornet]

Hmm, they should both be NAT B then
Try connecting one console to a mobile hotspot and try again
Do keep an eye on the support code for the connection test, if it changes to 22x-00, 22x-01 or 00x-99 then you cannot try the mobile hotspot, as then it would be NAT incompatibility

[wispy quiver]

Could I change it in my router settings?