vpn router

Dear Community, I try now for some time to connect to my home-network with an Android-Smartphone by establishing a VPN-tunnel. Somehow it doesn't work and therefore I hope, you can help me. I have an Asus RT-AC88U with Merlin Firmeware version 384.7_2. My Network-Setup is the following...

I am hoping to get a little help from the forum selecting a VPN router for my office. I have a wired gigabit network with a Qnap TS-559-Pro II that I use as a file server. I want to be able to access the qnap from my home and on the road. Special Considerations. Prefer a standalone appliance...