Network missing from sidebar(s)

I used to be able to transfer files from my 2018 Mac mini (Big Sur) to my 2015 MacBook Pro (was Big Sur, now Monterey) or vice versa, from either machine by clicking the name of the other in a Finder window sidebar and using Connect As to allow access. I don't need to do this often so I think the last time it worked they may have both been on Big Sur. If relevant, my Mini connects to the network by Ethernet and the laptop by wi-fi.

I now find that neither machine shows the other in the sidebar (that option is ticked in Finder Preferences on both) as is File Sharing. Neither machine shows the Network icon except occasionally and even then, if I click that, no other machines show on the network.

I'm not aware of changing anything except upgrading the laptop to Monterey.

Any suggestions?