How to register an external static ip on the server?

Question

linux

There is an Internet with a static ip-address through an adsl modem. The external address assigned by the provider is 80.240.74.xxx, the internal address on the modem is 192.168.10.10. You need to somehow register this external address directly to the server with CentOS for eth0. When there were several external IP addresses, IPADDR=80.240.74.xx2 NETMASK=255.255.255.248 GATEWAY=80.240.74.xx1 was specified in the CentOS settings for eth0. Now the provider has left only 1 external address, and this address should have access from the Internet not to the modem, but directly to the server. Port forwarding is not an option.
PS I'm not a system administrator and not a computer scientist in general, I just understand something. Here is the guide asking for help. And our system administrator is on vacation and not available.

Answer 1

On the router (adsl modem), you need to add your server to the DMZ zone. As a rule, this functionality is called (DMZ).

Answer 2

Router (modem) in bridge mode.

Answer 3

As written above, the adsl router is in bridge (bridge) mode. In Centos, configure eth0 as WAN IPADDR=80.240.74.xx2 NETMASK=255.255.255.248 GATEWAY=80.240.74.xx1, eth1 as Lan IPADDR=192.168.10.254NETMASK=255.255.255.0 (or easier Prefix=24) GATEWAY8=192.16. 10.254.