If your postage option is with DHL, you can track it here.
Otherwise your router will be dispatched from the UK by the Royal Mail, the UK’s largest postal service. You can track your router when it is within the UK shores here.
If you live outside of the UK, it will be passed to the Royal Mail’s partner in your country. Usually this is your largest / national postal service. If you are in the USA, your parcel will be delivered by USPS. You can track your router, once it has arrived State side, here.
For every other country, you can find the Royal Mail delivery partner using this page.