MacBook Pro can't connect to Time Capsule

My system reports this morning that I haven't backed up to my Time Capsule for 23 days. That would be the time that has elapsed since I took the TC offline and put my new Comcast gateway in service. So I really need to figure this out ... without buying a new backup drive.

I've used an Apple 2TB Airport/Time Capsule for a couple of years. It's handled my WiFi network and my Time Machine backups without a problem. Then along comes Comcast with their shiny new gateway device that combines modem and router. They promised "blazing fast Internet," so I went for it. About three weeks ago, I took the TC/Airport off-line and set up the new gateway. Till now, I simply let the Time Machine sit idle, and it's been 23 days since my last backup. I saw this week that El Capitan is available, and I want to get it ... but not before I have a solid recent backup!

I've researched some Community discussions that seemed to apply, and I've learned a few things such as "bridge mode." Still, I can't get my Mac to connect to the TC. So far, I've reset my TC with help from a post from an Australian gentleman who calls himself LaPastenague. Then I ran an Ethernet cable from the WAN port on the TC to a LAN (Ethernet) port on my gateway. (To be precise, I connected it to a WiFi range extender. The gateway sits in my living room at the other end of the house, but I doubt this arrangement has any bearing on my immediate problem.) LaPastenague (see Setting up Time Capsule in Bridge Mode?in this Community) posted some very helpful screen shots from Airport Utility. However, when I open Airport Utility, I don't see anything that resembles those screen shots. Here's what I see—

This morning, I clicked the Time Machine icon >> Back Up Now, which got me a notice that my TC is "not available." I clicked Details and got this:

I don't know what to do from here. I hope someone in this community can take me by the hand, so to speak, and walk me through the steps I need to take so I can get back to backing up. Thanks!