If it helps, the different colour LEDs are explained here:
https://community.sunrise.ch/d/30799-connect-box-3-hfc-technical-details-and-explanation-of-status-lamps
When I previously had a connection issue, I called the hotline and they sent me a new modem (including power supply and cables). If the issue is with your hardware then that will fix it. It only took a couple of days to arrive.
While on the call, they can also run some diagnostics which is useful, as the issue may be elsewhere.
I’ve had red flashing before (cable issue) and blue flashing (connection just about OK but not quite good enough for Sunrise to authenticate my modem), but never yellow flashing, so I don’t know what that really means!
You could try unplugging the cable at both ends, making sure that if any end has that central pin in it that it’s straight and centred, then plug the cable back in again.