As far as I can see, there could be at least a couple of reasons for this kind of behaviour. Both not related to NethServer, which is quite… picky to gave access to red-zone-connected devices (unless you tell/setup it to allow connection).
And about misdeeds of ISP, i would like to tell tales about one of the first FTTH ISP in Italy which connected all devices of the customers into one, single RFC 1918 class wide the whole city, with a maximum of 3 devices connected to ever router.
If someone’s asking “what about NAT?”, my answer is “I also would like to know…”
Or one of the most… “nice” (not the funny meaning) behaviour of one of the biggest consumers ISP’s in Italy, which is using Class-B addreses for routing the packets outside the consumer’s routers…
A little Traceroute from my laptop. My network is the row 1, from row 2 it’s outside the WAN interface of my router (which declare to have a public ip address)
traceroute 8.8.8.8
traceroute to 8.8.8.8 (8.8.8.8), 30 hops max, 60 byte packets
1 172.31.110.1 (172.31.110.1) 1.858 ms 2.079 ms 3.686 ms
2 * * *
3 172.17.89.189 (172.17.89.189) 31.188 ms 172.17.88.214 (172.17.88.214) 32.096 ms 172.17.89.57 (172.17.89.57) 33.641 ms
4 172.17.88.169 (172.17.88.169) 35.075 ms 172.17.88.153 (172.17.88.153) 38.829 ms 172.17.88.181 (172.17.88.181) 40.048 ms
5 172.19.241.101 (172.19.241.101) 45.845 ms 45.567 ms 172.19.241.113 (172.19.241.113) 45.003 ms
6 * * *
7 72.14.195.206 (72.14.195.206) 34.923 ms 74.125.51.12 (74.125.51.12) 35.314 ms 74.125.48.192 (74.125.48.192) 34.154 ms
8 108.170.245.65 (108.170.245.65) 36.844 ms 39.533 ms 108.170.245.81 (108.170.245.81) 41.188 ms
9 108.170.234.247 (108.170.234.247) 43.466 ms 108.170.234.101 (108.170.234.101) 43.535 ms 209.85.248.163 (209.85.248.163) 43.576 ms
10 google-public-dns-a.google.com (8.8.8.8) 45.925 ms 30.597 ms 30.842 ms