I’m really struggling to understand the SIM side, the page talks about 5G while tethered to a phone?

I think these models can act as router while leveraging RNDIS tethering if you don't have a separate sim card...

Zero mention of anything other than WiFi on its tech specs page :-/

https://store.ui.com/us/en/category/all-wifi/products/utr

This is incorrect. No sim support here, and no cellular modem either :-)

I was talking about the https://www.gl-inet.com/products/gl-e5800/, it in fact does have eSIM and 2 physical SIM slots.