So I've reconfigured SCOM (Administration -> Settings -> Security) and in addition to the "Review new manually agent installations in pending management view" I've check the "Automatically approve new manually installed agents". Then I've restarted the SCOM HealthService on the SCOM Management Server (net stop/start healthservice)
After the restart, when I opened console the previously pending servers were now in the "Agent Managed" folder (Not monitored, at the begining, but Healthy after few minutes when SCOM get the status from them).
For Security purpose "Automatic approval" should be disabled but to solve the issue you can enable it.