You're quite close as you have already installed @types/simple-peer
Now try creating a declaration file named typings.d.ts in the src folder or whatever the root of your project is.
Then type the following in the newly created typings file:
declare var SimplePeer: import('simple-peer').SimplePeer;
In main.ts, place the following line at the top of your imports (just to keep it separate)
///<reference path="typings.d.ts"/>
In your ts component, you should be able to import (etc):
import { Instance, SignalData } from 'simple-peer';
In my case, I'm only using Instance as other usage just didn't make sense when strongly typed. So, I have peer as a type of Instance as shown:
peer: Instance;
Now you should be able to explore your options via intellisense.
Let me know if this still didn't work so I can check properly as I implemented this a while back.
Also, if you managed to improve usage, share such improvements.
Hope this helps.
Thanks
For more information on declaration files:
https://angular.io/guide/typescript-configuration#typings