Replace Arris surfboard modem with SB8200, now Airport Extreme flashes yellow

FWIW. The Arris SB8200 is only a modem, not a combination router & modem (aka, a gateway) device. As such, if your networking goal with your AirPort Extreme/Express is to use it as your "main" Internet router, then it should NOT be configured as a bridge. Instead, it should be configured in its default router mode.

Typically, when changing network hardware, it is always a good idea to perform a complete power recycle of your hardware to get them to communicate properly. This includes doing a "factory default" reset on the Extreme/Express.

How to reset your AirPort base station - Apple Support

Once the AirPort Extreme has been reset, you can set it up again using the same name and passwords.

But.....before you do this, connect your Mac directly to the modem and make sure that you can get a good Internet connection that way. If you can't, there is a problem with the modem that will need to be resolved.

Once you have verified that modem is working correctly, power it off, then connect the AirPort Extreme and power it off as well if it is still on. Wait a minute, and power up the modem and let it run for a minute, then power up the AirPort Extreme and set it up again.

If you have not reset the AirPort back to factory default settings and then then set it up again, the AirPort is remembering some settings from your "old" modem, which is no longer there.

The title of your post indicates that you have an AirPort Extreme, but the text in your post indicates that you have an AirPort Express. Which AirPort do you have?

Open AirPort Utility on your Mac

Click on the AirPort Extreme

A smaller window will appear with information about the AirPort Extreme

Look for the Status setting

Click on the amber dot and another message will appear

What does the message say?