I have a very similar experience with Vodafone and Huawei CPE Pro: It occasionally switches down to 4G and doesn't go back to 5G. I haven't tried factory-resetting it but I found this one weird trick that I still don't understand (if anyone can explain or debunk it, that'd be great!):
When the router only connects to 4G (even after rebooting), trigger a network search by switching the network search mode from auto to manual. This works almost every time and you can do it either via the web interface or the mobile app (AI Life on Android):
- Web interface: Network Settings -> Mobile Network Searching -> Manual -> Save -> Confirm
- AI Life Android app: Show more -> Advanced Settings -> Carrier -> Network search -> OK
Once the list of found networks is shown (takes ~1min), just cancel/go back to revert the network search setting to "auto-select". Usually, I'm back to a 5G connection at that point.
Let me know if this works/fails for you