ARP isn't generally used the way you described. Per the spec, to resolve an IP address into a MAC address, you broadcast an ARP request for the IP address, and receive a unicast response from the host whose address it is - not a router. It also only resolves a single IP address in one request; it doesn't return the contents of the entire ARP cache.
If you've come upon this page because the ESP8266 on your (WeMos) Mega wifi won't connect to your router (etc), check the age/security of your router:
Wifi stopped connecting (Wemos D1 Mini)
This line of code that allowed connecting with my old router:
which also highlights that I should upgrade it...