Hello @Suryaprakash
It is possible to set a static ip on the WAN. Go to: Hardware - GX50 - Settings - IP configuration
Leave VLAN blank and enter the IP for the GX50, the Gateway-IP and the DNS.
On the LAN side the GX50 always has a preset IP-address (x.x.x.1), not sure if you were talking about the GX50 LAN-IP or how to set Reserved IP addresses.
Infos can be found here Meraki Go - Local Network Addressing - Cisco Meraki
Cheers