A comment by Hackintosh HD
Thanks! Just as a service for everyone else, here's the (rather hidden) URL to the GL.iNet modem firmwares: http://download.gl-inet.com/download/RM520GL-modem_firmware
As a sidenote, there are - or at least were - obviously provider-related hurdles for 5G use with Vodafone CallYa SIM cards which is a secondary troubleshooting path I'll follow: https://www.heise.de/ratgeber/Kein-5G-mit-CallYa-9529136.html