ERR_SSL_KEY_USAGE_INCOMPATIBLE NETGEAR Nighthawk(R) X4S R7800

The manual written in 2017 may no longer be entirely accurate.  The RBR50 that I purchased in 2016 also had a Remote Management feature very similar to what this R7800 manual describes (although it was https if my memory is correct).  When the Orbi 'app' was released, Netgear removed the Remote Management feature.  When the Nighthawk 'app' was released to replace Genie, perhaps the same thing happened to this product.  Only someone with access to an R7800 can verify what the current firmware does.

My sense is that the OP was not talking about Remote Management.

I still think that the issue is caused by how modern web browsers are almost paranoid in protecting users:

• If they access an http web site, the browser screams, "UNSAFE" because http is not encrypted.  There is usually an option (sometimes called "Advanced") to ignore the warning.
• If they access an https web site with a self-signed SSL certificate, the browser screams "UNSAFE" because no certificate authority has vouched for the SSL certificate.  Once again, there is usually a way to ignore the warning. YEARS ago, Netgear actually registered SSL certificates for a number of URLs, including routerlogin.com, routerlogin.net, orbilogin.com, orbilogin.net, but when the SSL certificate expired, it was not renewed. (which makes sense because those URLs do not resolve to any device owned and operated by Netgear.)

I have not run Voxel in years.  (I LOVE what he has done, but it would be hard to comment on the forum when running third party firmware.)  My sense is that web browsers would treat Voxel's web interface the same as they do Netgear's.