First of all, assuming that K router is ISP router, we need to remove nat configs from K router and add static routes on edge routers. there is also a duplicate IP address on serial interfaces. To make this NAT lab to work, we need NAT static entry on BB-S similarly what you have on DC router. Since you have same Private range on left and right sides, we need to hide those behind Nat. you can add on S router following entry: ip nat inside source static 172.16.50.20 193.95.76.20.
you can test it using BB-K, an ISP router by pining Public IPs like 193.95.75.3 or 193.95.75.5 or newly added entry 193.95.76.20. You cannot reach Private range from Internet, so you cannot ping any 172.16.50.x.
Normally and technically, to reach private web server from the Internet, routers will have static TCP NAT entries translating port 80 of Private IP to Public IP port 80. Example on BB-S has ip nat inside source static tcp 172.16.50.20 80 193.95.76.20 80
Regards, ML
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