I'm surprised it let you do this. 192.168.0.0/24 and 192.168.0.0/18 overlap, a good router should balk at this.
/18 is a 2/6 split of the third octet, IOW 192.168.0.0/18 is 192.168.0-63.x. You have a really big network there, 16K hosts.
If you really want a /18 and want to use 192.168 for it while keeping 192.168.0.0/24, then you should make it 192.168.64.0/18.