I have an ongoing project regarding using mobile phones as a modem as an alternative to dedicated routers whether it is for a cheaper broadband option, primary source, balanced or backup.
iPhones can be tethered as a client or via the hotspot functionality via USB for PC/Router etc. Both TP-Link and GL.iNet provided travel routers worked well previously but these are now regarded as slow devices if considering 4G+ and 5G. Later GL.iNet routers (Slate and Beryl) are faster but iOS 14 caused issues regarding iPhones. Hence why I am going back to test again.
Compatible USB to Ethernet adaptors for iPhone and Android 11 can be very problematic.
An alternative is a Router that supports Client/Wisp and using the phone's WIFI Hotspot. Yes there is an overhead and the network purists will have a heart attack but it is a practical solution if internet use is basic. A secondary benefit is accessing the free minutes via Bluetooth at the same time. The router can be either a cheap travel router or a monster. IP issues can be an issue and mitigated with a VPN and I am finding Wireguard performance encouraging.
Just need to put man hours in to test currently available solutions. Others may be able to assist.
The reason I like the idea of mobile phones as opposed to mobile routers is that control will continue to be centralised and increasingly complex in future with UPF/slicing. The service providers will have far more control regarding how eMBB devices behave. If using a leading manufacturer (e.g Samsung S20 or iPhone 13) they are less likely to discriminate (just a theory).