Can I use my own extension cable(s)?

Yes. Bear in mind however that this will almost certainly reduce your overall broadband speeds.

If you have a master socket with a double socket especially for broadband you can plug your extension into that - you won't need to use a microfilter at all in this case.

If you have a master socket with only one socket which is shared with phone and broadband, you'll need to use a microfilter and plug your extension into that. Never filter at both ends of your extension lead!

Best broadband speeds are always obtained by plugging your router directly into your master socket.