Hi Guillermo,
Thanks for reaching out, and sorry for the delay. DHCP should not be the issue. Did you register your phone with RingRoost already?
Instructions for registering a generic SIP phone are here:
https://www.ringroost.com/how-to-set-up-and-register-a-sip-phone.php#step_4_1
In order to register the Cisco SPA508G, the process is the same, but the interface is a little different- see the below screen shot of how to register the CiscoSPA508G with RingRoost (note: replace "SIP USER" & "SIP PASS" with the credentials on your RingRoost PBX "SIP Phone" control)