SB6121 has no built-in router (no NAT). DHCP server (192.168.x.x) only active when modem is booting up or no WAN connection. Else, only one device will be assigned WAN IP and WAN connection limited to that device MAC address (until reboot).
Ooma connected directly to SB6121 not receiving WAN IP suggests cable modem was expecting different MAC address. I know you said you definitely power-cycled, so perhaps glitchy modem or power-cycling not done in correct sequence.
@St3fx:
I have already asked you NOT to give any further 'help' in this matter - I was responding to another member and specifically I say to you, again:
You persistently re-post unhelpful info - I neither need nor want this.
Kindly desist.
The Ooma is now working and others asked questions which I can, and will answer.
Do not do this anymore.
Thanks.
Correction from my end to #22, above:
The Ooma box was too unreliable so we gave up on it & grabbed an Obi200 box when their pricing went back down, and since they are now heading for their official EOL, what may or may not become of GV itself and/or connecting it to a plain old type of phone becomes an even more curious subject.
What with ~2 years of possible service remaining - and the VOIP marketplace having shrunk so very much, it looks like getting by with a good provider such as VOIP.MS will become the best option once Obihai itself is EOL & maybe GV will be as well...?
And as to the Ooma stuff - nothing about it warranted sticking with it;
Its h/w, servers & (non-)support cause me to be amazed at how that company is even still around at all.