This seems pretty likely, but shouldn't a tech roll have included sampling the signal strength/snr at these frequencies? Or would the tech tools be likely to be on old frequencies too?
Tech should plug the meter into where the modem is too, though, right? And if the SNR looks good at the dmarc and not at the modem, there's your problem... Tech can peace out if you don't have inside line coverage.
or, plugging your modem into your dmarc/outside box for a little bit would also confirm or deny this