You are working on an informatic project and what is needed most in informatic projects is informtation:
software-serial works only reliable on rather slow baudrates on atmel chips
your codes says
Atmega 328P on breadboard
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what frequency?
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What oscillator?
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Can you give some hints where in your code you are sending the mac-adresses?
The more informational details you provide the faster you will receive help
That is the reason why this is important to read
best regards Stefan