Attention: the RT-AC88U (as well as the other ASUS routers) are not modems, but pure WLAN routers. In my opinion, Sunrise comes with a WLAN router including modem. With UPC you can configure it as a modem, which is not possible with Sunrise. If you now want to connect an ASUS router to it with Sunrise, it is theoretically possible, but it is not practical (look on the Internet under "Double-NAT") and tends to slow down the connection.

Test the mobile and wired internet at http://speed.sunrise.ch and contact Sunrise if you do not achieve the promised values/throughput. They can analyse the line better.

Have cablecom,works great but require a separate modem,not everything with tv box 2 room solution.
And set the modem as a modem not as a router then it works.

To the first question whether this router speeds up the internet...
rather not,the speed depends on the provider,location,user..etc.

And yes the router should work with any provider.