I'm assuming this is a routed client with a WiFi connection from the wan network to the main router. Some instructions have you create a new network "wwan" or similar but that isn't necessary, you can use the default wan just change it from wired to wireless.
Go to network-interfaces-(your WAN network), right below where DHCP Client is set there is a box to enter a hostname. This hostname should register in the main router when the wan gets its IP address.
Restart the network or just reboot to make sure that a new DHCP has been done, then from a machine on the main router's network, try to ping this hostname (pinging the router's own hostname from the LAN side is trivial, you've already done that). The main router may require a domain such as .lan or .local.
Once that works you can open firewall ports on your routed client to allow administration from the main network. Those would be port 80 (TCP) for http and port 22 (TCP) for SSH. By default OpenWrt blocks everything incoming to wan except pings.