Did you remember to change the settings to "point" your Mac to the Time Capsule as a backup destination in Time Machine Preferences?
If not, Time Machine is still remembering the "path" from your Mac to the Time Capsule for your old network setup, and you have changed the "location" of the Time Capsule on your network, so the Mac does not know where to find it.
Open Time Machine Preferences on your Mac (System Preferences > Time Machine) and delete the current setup for Time Machine by removing the Time Capsule disk.....likely called "Data"...... as the destination drive. Restart your Mac and set up Time Machine again.
Please remember that what you are attempting to do is not officially supported by Apple, so while it will probably work, there will be no support from Apple in the event of problems with the setup.
High Sierra has recently updated security on the Mac operating system, and we have seen some reports of "lost" Time Capsules as a result, so that might be a possibility as well.