Learn Swedish: Routern fungerar inte, men han kommer att starta om den. - The router does not work, but he will restart it.

Starta om (to restart) is a particle verb. In Swedish, you cannot split a particle verb with an object pronoun like den. The pronoun must come strictly after the particle, so it is always starta om den. This is different from English, where pronouns often go in the middle (like 'turn it on').

Why do we use den at the end instead of det?

Den refers back to routern (the router). Because router is an en-word (which we can see from its definite ending -n in routern), we must use the en-word pronoun den to say 'it'. If the noun had been an ett-word, we would have used det.

Could we use ska starta instead of kommer att starta here?

Yes, but the nuance changes slightly. Kommer att is the most neutral way to express the future in Swedish, simply stating what is going to happen as a fact. Using ska would put more emphasis on his personal intention, duty, or a firm promise to fix it.

How does the English loanword router take Swedish noun endings here?