Accessing orbilogin.local shows an error "certificate is using a broken key size" in Safari MacOS

I think there may actually be two different issues here.

First, orbilogin.local doesn’t appear to be a generally supported access method on current Orbi models. I tested it on an Orbi 970 from both macOS and Windows, and it doesn’t resolve at all (ping orbilogin.local fails, and Bonjour/mDNS doesn’t advertise that name). However, orbilogin.com, routerlogin.net, and the router’s LAN IP all work normally. So you could use https://192.168.1.1, https://orbilogin.net, or https://orbilogin.com to access the router, albeit going through the certificate warning first.

Interestingly, Netgear’s documentation almost always references orbilogin.com and routerlogin.net. I searched netgear.com via Google, the only Netgear.com reference I could find on the whole site that mentions orbilogin.local is for the Orbi 770 series, not the 870 or any other router. That makes me wonder whether .local is only implemented on the 770 - or that even that reference could be an error. Since ceebee6cc​ has an 870 system, and mine is a 970, I really don't think orbilogin.local is supported at all in those routers' firmware.

The certificate warning is probably a separate issue. As others have implied, current versions of Safari are much stricter about older self-signed certificates than they used to be. I also receive a certificate warning when accessing my Orbi over HTTPS by IP, orbilogin.net or by orbilogin.com, but after accepting the warning the admin page loads normally.

Does https://orbilogin.com, https://routerlogin.net, or https://192.168.1.1 work for you? If they do, I’d be inclined to ignore orbilogin.local unless Netgear specifically says the 870 should support it. I don't think it does.