CC2531 Dongle features Texas Instruments CC2531 SoC and is used as coordinator for Zigbee2MQTT.
If you wish to extend your network, you have two options:
1. You should have one or more router devices. A router device is every mains powered ZigBee device such as Ikea Tradfri bulb or Xiaomi Wall Switch.
2. Install second(or more) CC2531 flashed with router firmware
Please note: CC2531 with router firmware is used only as a repeater!