I’m not sure if you still need help, but I just so happen to have both a ZBBridge and a Tradfri repeater.
You can’t pair an IKEA device with a Sonoff ZBBridge on factory firmware. And you cannot add IKEA devices through the eWeLink app. If you were to flash your bridge with Tasmota, then you could add the repeater through ZHA, Zigbee2MQTT, or Tasmota.
Personally, I don’t recommend flashing Tasmota or going down this rabbit hole. I’ve done it and the Trader repeaters just don’t play well with any zigbee configuration I worked with. They constantly drop packets, go to sleep, stop responding. They’ve been nothing but a pain in my backside. I’ve updated firmware on them, tried different placements, Tradfri repeaters are just junk imho.
If you want a repeater and are going to use the ewelink app, look for a zigbee smart plug that has “e support” or supports eWelink in the product description. Most smart plugs act as decent zigbee repeaters.
P.S.:
If you want these Tradfri repeaters to pair, grab a sim ejector tool and insert it in the small hole next to the USB port. Feel for a button, you might miss. Press the button for 3 seconds and the LED light will lightly dim/pulse when it’s in pairing mode. Then set your hub to pairing mode. (Again, this will not work in the eWelink app.)