Unable to open ports or create vpn

Sorry if this is in the wrong forum, I put it here since it seemed to have elements of questions from multiple networking forums.

I seem to be having an odd issue with ports and VPNs, and I really cannot figure out what is causing it.

Short backstory: A year or so ago, when Verizon was my ISP, I ran a Minecraft server with a Hamachi VPN. I had looked into forwarding ports at the time, but decided to go the VPN route because I wasn't completely sure how to assign a static IP to a computer on the network. I had absolutely no issues setting up the VPN or hosting a server on the network.

Recently I decided to give port forwarding a go again. I successfully assigned the static IP and put the port number into my router's port forwarding settings, but the port was still closed. I tried using online port checker tools and having friends try to log into the Minecraft server. No luck.

After reading multiple port forwarding tutorials to make sure I had everything correct and double- and triple-checking all my settings, I began to suspect that something else was causing the problem. I have an older router set up as a repeater on the network so my family can connect to the Wifi network from anywhere in our house. Thinking maybe the repeater or even the main router itself was to blame, I decided to try another router. After disconnecting the current routers and setting up a spare router I had, I tried everything again. Still no luck.

At this point I was frustrated with spending so much time on this, so I gave up for a few weeks before going back and doing some more investigation. I tried opening different ports unsuccessfully. No port I tried opening would work. I decided to give in and just set up Hamachi again, since that had worked flawlessly before. However, after setting up a Hamachi VPN, I realized people could join the "room," so to speak, but could not connect to the VPN itself. Even hosting a Minecraft server over Hamachi wasn't working this time.

Assuming it was just an issue with Hamachi, I tried another VPN software. I got the same error - people could log in, but not connect to the VPN. Thinking back to the port checker telling me port 80 was closed, I pretty well gave up.

The thing that confuses me is that I can explain to others how to do the exact same things I'd like to do. I've walked one friend through setting up a VPN the exact same way I did, and it worked on the first try. I walked another friend through assigning a static IP and forwarding ports. I don't know what's different about my network that won't allow either of these things. I've searched my router's configuration for any setting that would override port forwarding or block a VPN, but I can't find anything.

I've suspected that Frontier, who bought the Verizon customers in my area, may be blocking these things on their end. I've talked to a friend of mine that's taken networking classes, though, and he's told me that's impossible.

At this point I've run out of ideas as to what could be causing the problem. Any help would be greatly appreciated.

The modem (supplied by Frontier) is a Westell Model 6100G, the router is a Medialink MWN-WAPR150N.