FYI; Zigbee Router firmware for EFR32MG21 adapters has now been released by xsp1989 on GitHub.
From readme it sounds tested with ITead Zigbee 3.0 USB Dongle and an SM-011 based USB adapter.
The same “SM-011 V1.0” Zigbee radio module design by CoolKit Technologies is also used inside some Zigbee gateways/hubs like the popular ITead Sonoff ZBBridge and ZB-GW03 eWeLink Ethernet Zigbee Gateway sold by EACHEN and SmartWise, so could perhaps be that the same Zigbee Router firmware could maybe also be used on the SM-011 Zigbee module in all or some of those products as well?
https://github.com/xsp1989/zigbeeFirmware/tree/master/firmware/Zigbee3.0_Dongle/RouterForDongle