Hi SFM,
Welcome to the forum.
Almost any router/modem can be configured to work - you need to ensure that it allows "port forwarding" to be configured. Some do this explicitly, others are configured in two stages. I haven't used either of the two you mention, but in My Netgear DG834G, it needs to be configured in two stages, firstly to define a "service" which identifies a port number or range or port numbers for a specific purpose, and then set up a "firewall rule" to divert incoming traffic for the specified service to a predetermined local address.
It's also very useful if you can define a fixed local IP address for the PC running Broadwave.
If you can listen on another PC on the same network, it confirms that Broadwave and that PC are set up correctly, you just need to consider the router.
Note that some home routers do not support "NAT Loopback". That means that you cannot specify your external IP address locally - you can only use it if you are outside your own local network.
Also, note that some ISPs will block traffic on selected ports. The default port 88 used by Broadwave is blocked by some ISPs, particularly mobile data networks, so I recommend you use a different one, e.g. 8818 or similar number, which is not assigned to another service (as 88 is).
Going back to the routers you mentioned, have a look at the manuals and check you've got port forwarding set up correctly, and try a different port number too, just in case. If you have any questions on what I've written, just add a reply and I'll try to answer when I can.