Hmm after some searching I found some updated lua command
Replacing the UserParameter line in /etc/zabbix_agentd.conf.d/network
with the string from this page
https://github.com/openwrt/packages/blob/master/admin/zabbix/files/network
UserParameter=netowrt.discovery,lua -l ubus -e 'u=ubus.connect();list="{\"data\":[";dump=u:call("network.interface", "dump", {});for _, intf in ipairs(dump.interface) do list=list.."{\"{#IF}\":\""..(intf.device or intf.l3_device).."\", \"{#NET}\":\""..intf.interface.."\"},";end;list=string.gsub(list,",$","");print(list.."]}")'
I restarted the Zabbix agent on the router. Then I deleted and re-added the router Zabbix. I had to enable and re-enable Network Interface discovery as well after that, but now I have traffic graphs.