Why not just get a new router if you can't reflash yours? I don't see how they would hack the router you have now unless it has publicly known exploits. They aren't just going to be able to hack your router unless they have an exploit for it, the most they could do is use an exploit in the Wi-Fi Security Protocol you have, and then they would only have access to your Wi-Fi network, nothing major.
Trust me, if they can remotely flash firmware to your router, your router has exploits that are known or they found. Also, are you saying these people are remotely installing a VPN on your phone or something?