internet interface card, what is the max download speed

Your problem is not the phone.

Rather, your problem is the WiFi router.

You need to get one that supports at least 1900mhz frequency on the AC spec.

https://www.amazon.com/TP-Link-Wireless-...

The AC standard will let you run as fast as your Fiber connection is, with perhaps a 10-20% speed degradation due to environment.

Also, the speed metrics may be a bit deceiving.

Speed on the Internet service is measured in Mb/s.

Download speed on a Browser is measured in MB/s (note b vs B).

There are 8 bits (b) in a Byte (B).

So you will have to calculate speed by dividing 250 million bits by 8 and possibly deducting another 10-25% (environment) to get the speed you theoretically should be getting.

So theoretically, you should be getting 27-31MegaBytes per second download speed.