Here is an example of 3 methods, function with Javascript, change with JSONata and a low code option with split and join.
hope it helps
[{"id":"d6cf43f3.1a4c08","type":"inject","z":"bf9e1e33.030598","name":"","props":[{"p":"payload"},{"p":"topic","vt":"str"}],"repeat":"","crontab":"","once":false,"onceDelay":0.1,"topic":"","payload":"[{\".id\":\"*3B\",\"interface\":\"wlan1\",\"mac-address\":\"xx:xx:xx:xx:xx:xx\",\"ap\":\"false\",\"wds\":\"false\",\"bridge\":\"false\",\"rx-rate\":\"54Mbps\",\"tx-rate\":\"65Mbps-20MHz/1S\"},{\".id\":\"*3x\",\"interface\":\"wlan1\",\"mac-address\":\"yy:yy:yy:yy:yy:yy\",\"ap\":\"false\",\"wds\":\"false\",\"bridge\":\"false\",\"rx-rate\":\"54Mbps\",\"tx-rate\":\"65Mbps-20MHz/1S\"}]","payloadType":"json","x":160,"y":1960,"wires":[["9cb4a346.0e272","8f21907.5bde07","53a4ea5.a66fb14"]]},{"id":"9cb4a346.0e272","type":"function","z":"bf9e1e33.030598","name":"","func":"msg.payload = msg.payload.map(obj => obj[\"mac-address\"])\nreturn msg;","outputs":1,"noerr":0,"initialize":"","finalize":"","libs":[],"x":360,"y":1960,"wires":[["51a7f44d.0d30ac"]]},{"id":"8f21907.5bde07","type":"change","z":"bf9e1e33.030598","name":"","rules":[{"t":"set","p":"payload","pt":"msg","to":"[$$.payload.`mac-address`]","tot":"jsonata"}],"action":"","property":"","from":"","to":"","reg":false,"x":410,"y":2020,"wires":[["51a7f44d.0d30ac"]]},{"id":"53a4ea5.a66fb14","type":"split","z":"bf9e1e33.030598","name":"","splt":"\\n","spltType":"str","arraySplt":1,"arraySpltType":"len","stream":false,"addname":"","x":280,"y":2100,"wires":[["ec74879d.e89d18"]]},{"id":"51a7f44d.0d30ac","type":"debug","z":"bf9e1e33.030598","name":"","active":true,"tosidebar":true,"console":false,"tostatus":false,"complete":"false","statusVal":"","statusType":"auto","x":660,"y":1960,"wires":[]},{"id":"ec74879d.e89d18","type":"change","z":"bf9e1e33.030598","name":"","rules":[{"t":"set","p":"payload","pt":"msg","to":"payload.mac-address","tot":"msg"}],"action":"","property":"","from":"","to":"","reg":false,"x":460,"y":2120,"wires":[["6a5ec440.d583b4"]]},{"id":"6a5ec440.d583b4","type":"join","z":"bf9e1e33.030598","name":"","mode":"auto","build":"string","property":"payload","propertyType":"msg","key":"topic","joiner":"\\n","joinerType":"str","accumulate":"false","timeout":"","count":"","reduceRight":false,"x":650,"y":2100,"wires":[["51a7f44d.0d30ac"]]}]