Go into your router settings and make notes of the crucial data you need to handshake with your service provider. Disconnect that old router and throw it away. Buy a good one and install it then change the log in details from the generic ones that it comes with, for a complex username and password.
Set up an equally strong wifi security key including upper and lower case letters, numbers and $%£ symbols.
Change both the access password and the wif-fi security key monthly. Ity will make things harder to access.