On the product page (Clipper HAT Mini (LTE 4G for Raspberry Pi)), speeds of up to 5Mbps (uplink), and up to 10Mbps (downlink) are advertised.
However I don’t see how that’s achievable over a serial connection. The maximum speed I was able to configure it is with a baud rate of 921600. I tried recompiling pppd to be able to set the baud rate at 1842000 and 3686400 (as reported by AT+IPR=?) but it seems to perform worse than 921600 (e.g. only one ping out of 3 comes back if the baud rate is 1842000 or 3686400 when it works reliably at 921600). I suspect some flow control issues, and since the HAT is not exposing hardware flow control pins there is now way to fix this.
The obvious solution is to switch to the USB interface, sadly it is not routed on this HAT.
Do you have any suggestions to achieve better speeds? Thanks!