Even though your router is already on a non-DFS channel (44), the issue may still come from how the channel width is negotiated.
Many ISP-provided routers set to “20/40/80 MHz (auto)” will actually fall back to 20 MHz due to interference or firmware limitations, even if the Ul suggests otherwise.
This would explain the ~20 Mbps cap, as the connection is effectively stuck at 20 MHz.
I would recommend forcing the channel width to a fixed value (40 MHz or 80 MHz) instead of auto, and setting the channel manually.
If possible, testing with a different router would also help confirm whether this is a router-side limitation, especially since your connection works fine with a phone hotspot.