In that situation, I would put the vendor modem in a microwave or other impromptu faraday cage to prevent the leakage. Remove/isolate the antennas as best as possible.
Those vendor modems are rentals and expected to be returned in working order. Would you likely get away with it? Sure, nobody is paying techs to diagnose why the WiFi is failing for unit #367326, but cutting traces is definitely crossing some lines.