I have an ASUS router with a built in VPN server. Have used that for some time and no problem accessing from a windows computer or my Android phone.
Just recently got an IPAD. Trying to set up access from it. Not having much luck. My router seems to be only PPTP. In my IPAD I don't see that option. Tried the other options, but they don't seem to work.
All my VPNs are L2TP over IPSEC which is what apple devices want.. Super easy to setup on Ubiquiti Unifi gear... It might not be supported on most consumer grade gear...
OpenVPN will be quite slow on a home/small office router. Sometimes just a couple of Mb/s thruput when connected via VPN. The less encryption (PPTP for example) the faster it will run.
I’ve tried OpenVPN on my Synology before, but it’s an old DS115 and doesn’t work too well for the task. Very slow like you experienced with your router.
Since I have an always on computer for other node.js applications, I just use TeamViewer, which for personal use is free. Then open a browser on the remote computer and access my hubs. This allows access from my phone and a laptop with relative ease, and the speed is just the same as if I were at home.