Change the public ip of my server connected to two modems

Hello, I am new to networking, I have two internet modems connected to a swith tp-link, the problem is that when I upload a page through no-ip and assign the public IP address of my server it does not work because ip public address changes constantly because the switch assigns a different ip given by either modem, someone knows how I can restrict so that the IP address published only assigned a modem?