RM500Q-GL with current qmi_wwan in v21.03.2 reduced downlink speed

I managed to integrate a Quectel RM500Q-GL in an Raspberry Pi 4B running OpenWRT 21.03.2. Modem goes online and I have a connection etc.
However, testing the speed I have the following phanomenon:
DL ~1 Mbit/s
UL ~7 Mbit/s
This is reproducable. Test has not be done via WiFi, but directly on the Pi. So it is the pure WAN speed of the modem.
If I test at the same spot with my mobile phone, I get +10 Mbit/s in DL. Any ideas what could be hindering downlink speed?

Best regards,
Mike