Hello Robert,
Would it be possible that you would like to see my picture again if I have filled in everything correctly?

Yes, sure. But at the moment all works fine in your website, you should not change FB settings. All works correct. Also I see you've set redirection URL to /forum/ this is the same Forums option in selector:

Profile | Forums | Custom

Hi Robert,
After research on the browsers below, we will not see the FB login.
It is about this ;
- Phone: Samsung Galaxy S6
- Samsung Galaxy tab A
See also the print screen
The A tab is less than six months old.
And all plugins on the phone and Tab have been updated.
Cashe is also empty.
----
We have recently updated this as you tipped us;
Also I see you've set redirection URL to / forum / this is the same forum option in selector: Forums.
----
It is remarkable that we do not see it through these two mobile units. But can be seen in a desktop windows 10  🙃 

I'm sorry but I have no idea why it doesn't work on these devices. This maybe even device specific issue, this may work on  same other device. FB button loads 3rd party SDK lib. Maybe your device stops 3rd party content loading or it blocks SDK or something else. That's very hard to debug. I recommend do this test on wpForo.com too. Also with the same other device.

This man has the same problem.
https://wpforo.com/community/how-to-and-troubleshooting-2/facebook-login-on-mobile-not-showing/#post-13157

I'm sure this plugin is not working properly, because I've tested it on several mobile devices today. Think that you still have to see if it is well built for mobile use.