I’m pretty sure that you’ve already checked, but the obvious things sometimes fly under the radar and go unnoticed: is the phone in file transfer mode in the first place? Other one (which has bitten me) is if you’re using an usb-hub, try direct connection and/or different ports on the host computer.
Personally I’ve spent far too long to try and hunt down something obscure while the fix was really simple as some default option changed with updates or whatever. And in general I’ve forgotten to check the simple things first way too many times and that has caused wasted hours way more than I want to count or admit.
The process is to go step-by-step. First direct connect to modem you have, bridged connection if possible, and test with multiple bandwidth measurements (speedtest, fast.com, downloading a big file from some university ftp…) and work your way downstream of the network. And on every step test multiple scenarios where it’s possible, preferably with multiple devices.
When I got a 1Gbit fiber connection few years back I got an Ubiquiti Edgerouter-X with PoE-options. On paper that should’ve been plenty for my network, but in theory with NAT, DNAT, firewall rules and things like that it capped on 6-700Mbps depending on what I used it for. With small packets and VPN it dropped even more. So now that thing acts as an glorified PoE switch and the main routing is handled with Mikrotik device, which on manufacturers tests should be able to push 7Gbps on optimal conditions. I only have 1/1Gbps, so there’s plenty of room, but with very specific loads that thing still is still pushed to the limit (mostly small packet size with other stuff on top of it) but it can manage the full duplex 1000Base-T. And on normal everyday use it’s running at 20% (or so) load, but I like the fact that it can manage even the more challenging scenarios.