Adding a router to Home Internet

If half can see the gateway and the other half can't, something else seems at play here.

First, what gateway replaced the Nokia? Did they give you the Arcadyan KVD21 5G Gateway?

Second, what devices aren’t connecting? I'm not sure what a Kobo is (an e-reader?), but make sure that you are using the same network name (SSID) that you were using with your previous system.

Third, after your create your primary WiFi network using WPA2/WPA3, create another one that is WPA/WPA2 and only uses the 2.4 MHz range. The SSID needs to be different from your primary network and it should not be hidden. Use this network for older devices or devices that are giving you problems connecting. Make sure your printer, as an example, doesn't have a static IP from your previous ISP.

If you're getting a good signal from T-Mobile and the gateway is connecting to some devices, just not all devices, then the above steps should resolve the problem. I have the Arcadyan KVD21 5G Gateway and I only receive a 4G LTE signal at three bars (Good). I have no trouble with any device on my network, and I have a mix of old, new, printers, smart switches, Macs, iPhones, iPads and several Echos. The only problem I had during setup was when I assigned a static IP to a device.

Post back how you made out and what router T-M sent you.