First to make an intro, the VVX 300, 310, 400, 410, 500, 600 are the "Gen 1" models and we are no longer actively supporting them, as I believe they are EOL from Polycom. This means that the last tested firmware is 5.6.0 and the configuration guide for them is here:
https://www.3cx.com/sip-phones/polycom-vvx-eol/
These should not be confused with the VVX 101, 150, 201, 250, 301, 311, 350, 401, 411, 450, 501, 601 which are the "Gen 2" and "Gen 3" models, that were updated recently.
Ok, so the good news is that the phone seems to be downloading the configuration from the 3CX server. Just to confirm, you are manually configuring the name right? Theses settings were taken from the PBX.
By the time this is setup via an SBC, the Outbound Proxy should be the Local IP of the SBC and the Server Address should be the FQDN of your 3CX PBX.
By the time the SBC shows up in the MC as green and the phone is not being connected, I would suggest maybe running I capture on the SBC itself and see if the Phone is sending REGISTER sip messages.