my asus router can handle 3.2 gig and my internet service speed is 400 mbs. But I only get 87 mbs

There is definitely a problem somewhere, but there are more questions than answers here. For example, how are you determining the speed? Are you testing with different devices?
Ultimately, you’ll need to do some troubleshooting. Here is my guide on how to thoroughly troubleshoot an Internet connection. https://marcelbrown.com/2015/08/13/is-your-internet-speed-not-what-it-should-be-how-to-test-and-isolate-the-problem/