There have not been any recent changes to the game itself. Checking the DownDetector website, there don't appear to be any major issues currently, though I'm not sure how popular the site is with PS4 users. The map has no key code for the colours used, but the chart should provide numbers for your region(s).

Some of this was posted in other topics, but:

Have you tried joining by invitation and in the lobby?
If the multiplayer setting is on invite only, try switching it to friends only or public.

Have you tried hosting/joining an Arena match?

Try having the person joining going into Arena mode, create an arena, then exit to the main menu and try joining in the lobby. Alternately, create a multiplayer game lobby, exit and try joining.

Have either of you tried playing with anyone else?

Especially if you can play with other people, but not each other, it may help to remove the game profile.
In Settings, Storage, System Storage, Saved Data, select the game, then select 'Profile', press the Options button and Delete. Just delete the profile slot to keep any saves.

Can you try switching either or both PS4s between wifi and wired, if available, to see if that makes a difference?
If possible, testing with a mobile phone's wifi hotspot feature, or other available network, could help confirm if it is a network or system issue.

Try clearing the PS4 cache: press and hold the PlayStation button on the controller, select 'Turn Off PS4', wait another 30 second after it finishes shutting down, and start it again.