You can’t just “swap” the ethernet cord from one modem to the next. Your account won’t run both at once. Installing a new one takes it to be initialized by the isp, the old one not active, then the internet setup.
So do this.
install the modem. Call the isp and make sure its working and they can ping it while connected to it. Once thats done. power it back off, (along with your router), Make sure nothing is connected to it. Boot it up. give it 5 minutes to fully boot. Make sure the internet icon turns on. If it does, plug the router your have (which is? ) into it and boot it up. give it 5 minutes to boot. Then run through its internet setup page. Make sure that you DO NOT connect anything else to the S33. It has more than 1 ethernet port but most ISP’s only support a single device connected because they only allow 1 public ip address.
Try that first.