Re the internet, have a look at the label on the back of the router and it should have the admin password on.
Open a web browser and enter 192.168.1.254, then give the details from the sticker when prompted. Once you’re in, you should be able to find where to change the WiFi password. Most importantly, if you succeed in doing this, also change the admin password on the router while you’re in there, even if you just add a character at the end, so that he can’t sneakily change it back while standing outside.
If he’s changed the admin password himself out of spite, you’ll need to do a factory reset of the router by pressing and holding the reset button on the back, which will make it go back to the password on the label.
While you’re at it, make a note of the WiFi password he changed it to, and see whether it unlocks your Amazon account as well: it’s entirely possible that he’s made the password something petty and that he’s reused it wherever he feels he’s able to annoy you. If he has and you can get back into your Amazon account that way, you can edit the email address in the account settings, and make sure to set up 2 factor authentication while you’re at it.
I don’t know whether he’s likely to know your passwords for email etc, but it’s probably safest to assume that he does, so next step is to change your email password if you haven’t already, as well as making sure you have 2FA on there for added safety. Ditto social media.
I know this is all overwhelming, but between us here we have more than enough knowledge and experience to help.