You have your DNS A record pointing to the public IP of your router and your router is forwarding TCP 80 traffic to the internal IP of your CentOS web server? You're doing it right!
A few thoughts though, you mentioned you cannot access your router (from the public IP assigned to the WAN port I must assume?) with TCP 80 being port forwarded to your CentOS web server. What make and model router do you have? Several routers let you change the management port so if you change the router management port from 80 to 8080, then you could access your router by public IP assigned to the WAN port by opening your web browser and pointing it to http://routerpublicip:8080 or http://yourdomain.tld:8080.
Ideally remote router management is disabled, but I understand you may want to have that enabled.
Do you have a static or dynamic public IP? If you have a static IP address then there is not a problem.
If you have a dynamic public IP, then when your public IP changes the IP that your DNS A record for your domain points to will not be the new IP address. In that case, you will want to setup dynamic DNS with namecheap (https://www.namecheap.com/support/knowledgebase/category.aspx/11/dynamic-dns, https://www.namecheap.com/support/knowledgebase/article.aspx/595/11/how-do-i-enable-dynamic-dns-for-a-domain). You will then either need to configure your router or a computer in your network to login to namecheap and update your A record any time your public IP address changes.