When a new client connects to my Sagemcom 5688W gateway (i.e. to the wifi SSID broadcast by the gateway), it takes up to 30 seconds for the client to connect to the internet. During this ~30 second timeframe, the newly connected client gets DHCP and it can ping the gateway, but it cannot connect to the internet. This happens on all clients regardless of the client OS. This makes it impossible for some clients to connect to my wifi SSID (specifically, neither of my printers will connect because they time out before the gateway allows them to connect after 30 seconds, so I have to print like a caveman via USB).
I tried factory resetting my gateway, no change. I called T-Mobile support, they sent me a replacement gateway, it has the same problems. In my home, I have a full 5g T-Mobile signal, no obstructions around the gateway, and no wifi interference (I have confirmed this with advanced wifi testing tools on Linux, and an Aruba UXI sensor).
Does your gateway have the same problem?