Home internet service IPv6 traffic is all filtered even when using a Netgear LTE router. No port forwarding. Plz fix!

I solved my specific problem, but it's not a generic solution for all.  For those that are interested, here's how I solved the double-NAT issue and got VOIP to work.

I got the TMHI trashcan gateway to use as a secondary WAN connection (my primary is Cox cable).  I'm using a Peplink Balance 20 router, which allows dual WAN connections.

Peplink also offers a service called SpeedFusion Cloud, which is basically a VPN.  However, it also offers advantages like WAN smoothing and the ability to selectively route your traffic over both WANs simultaneously.  This means you can pull the plug on one WAN and someone on a VOIP call won't even notice a problem (I verified this by actually doing it).

Since SpeedFusion is an outgoing connection that is initiated by the router, it doesn't care if it has a public IP.  It works fine with the private IP address assigned by the T-Mobile gateway.

This solution is a variation on using a VPN that others have mentioned.  The cost of the SpeedFusion cloud service is as low as $20 for six months of service.  Right now I'm only routing my VOIP traffic through it, since our VOIP service is critical and it is the only service that appears to be affected by double-NAT issues.  Also, it doesn't make sense to pay for the extra bandwidth just to stream Netflix over a redundant connection.  However, it might make sense to route business-critical Zoom calls this way.

For anyone using a Peplink router, this might be a good solution. Now that I can split my traffic across two providers, I'm considering cutting the speed on my Cox service.  The cost savings by doing this will just about pay for the TMHI service and the small extra cost of the Speedfusion Cloud service.  This means I will have fully-redundant internet service for about the same cost as my original cable service.