I’m thinking CGNAT too, though a 200. IP address shouldn’t be used for that AFAIK.
This indicates an upstream problem; nothing to do with your Neth installation.
In CGnat case, at least here in Brazil if that was not in contract you can “change” it back to regular dinamic ip! 
Just call your ISP and ask them to disable CGnat.
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Stupid question as I’ve never heard of it before but do ISP perhaps block port 22 like some block 25 probably might not be this but for ruling things out try changing the SSH port on nethserver and try again another thing to test for to ping.eu select port check and put your public IP in and port 22 and it will say if it’s open or not
Stupid question…
but it is normal that in your configuration the gateway is
192.168.1.0 ???
In my opinion the problem is there
Yes, it happends, ISPs here in brazil often blocks 25, 22 and 21!