It is working now.
on RISCOS forums they said that USB-to-Ethernet adapter that i see on my settings is the Ethernet. but because they Ethernet on the RPi 3 is USB based then that's why it appears this way.
Another point of confusion that happened with me is, after the RISCOS fails to get an IP address from DHCP, it falls back to manual IP address. Even If I try to reboot again, the IP assignment settings is set permanently to manual assignment!!! I assumed it should try DHCP every time it boots but it seems it is not. I had to go to the IP assignment settings and change it to DHCP to get it to work with DHCP server.
Now things are fine. thank you all for your valuable assistance.
Last edited by andalus
on Sun Nov 25, 2018 12:28 pm, edited 1 time in total.