Hello,
On which interface are you trying to get snmp replies ? Lan ? Wan ? Other (VPN) ?
If it is on wan you must allow it explicitly: go to Services->SNMP and set “Enable remote access”.
For snmpget / snmpwalk / … use a ssh or cli console and install snmp-utils:
libnetsnmp appears to be miissing install it with opkg install libnetsnmp.
Which firmware version do you use ? The current version contains libnetsnmp.so.40. Maybe you can try an upgrade ?