Mac – Time Machine can’t find Time Capsule – but can mount it

I have a 5th Gen (A1470) Time Capsule on our network. The WAN ethernet port is wired to a router. The TC has a static IP, Wifi is disabled (on the TC) and router mode is off (bridged).

I have a Macbook Pro (El Capitan) currently backing up to the TC using our router's Wifi (only 1.31GB after 20mins though, looks like it's stalled). I have a Mac Pro (El Capitan) that's found the TC and still preparing backups.

However, I have an iMac (Yosemite) on the WiFi network that cannot find the TC in Time Machine preferences. However it can connect to it and mount it using the Finder (the TC shows up under the Shared sidebar).

I hard reset the Time Capsule and reconfigured as above before attempting backups on all 3 devices.

• How come the iMac can't find the TC in Time Machine – yet finds it fine in the Finder?
• Why is the first backup so slow?

UPDATE: It appears Time Machine wants to own the network, so I have reenabled Wifi (as a new Wifi network since Time Capsule can't extend non-Apple Wifi, yes really) and the Macbook Pro is using that Wifi. Now it is backing up quickly. The Mac Pro has now also been wired (ethernet) into the LAN ports on the TC, instead of trying to access it via the router. It appears TC wants to be the hub/router instead of playing nicely as a network device. The other iMac is now on the TC Wifi network – but can't find the Time Capsule in Airport or Time Machine – go figure.

UPDATE 2: I have upgraded the iMac in question to El Capitan and applied the available updates. Problem persists and I cannot see the Time Capsule in Airport Utility or Time Machine preferences. All other machines on the network are backing up to the TC now either through ethernet or Wifi.