Unable to access ISP router from internal PC

I installed fedora35 as a NAT, mail, web, DNS server. There are two network cards, one is onboard and the other is pci, net.ipv4.ip_forward=1, DNS works fine. Without a firewall, selinux is disabled. I can access the internal and external ip from the server. But the internal windows PC cannot access the ISP or external IP. How should it be resolved?

Some time ago I read somewhere about a different behaviour in NAT/Masquerading introduced with firewalld-1.0.0.
Try to take a look to this blog post Firewalld Fedora 34 -> 35 Masquerade between Zones not working anymore | IT-Hure

Hi Alessio :

Problem has been resolved. Thanks a lot.

@kevinhsieh], if @alciregi Alessio’s response helped you resolve your issue, please click the Solution button under his message so that others know to look there if they have a similar problem. Otherwise please mention how you resolved your issue. Thanks