Before you replaced the old modem in September 2023, were you having any issues? It sounds like all your issues began with the first, new Cox modem. Did you contact Cox to have the new modem re-provisioned? Modems aren't my thing so I'm not sure if a replacement modem has to be re-provisioned. But it seems like a different type of modem would need to be re-provisioned for it to properly connect with Cox's network.
WiderMouthOpen is the forum expert on modems. The following is for him. Does each replacement modem have to be re-provisioned? Or is it only necessary when the replacement is a different type of modem? Does provisioning just identify the type of modem or the specific modem? If it's the latter, then I know provisioning is necessary each time a modem is replaced or it wouldn't work at all.
Also, are you aware of anything that could cause an incompatibility issue for the TM3402 that wouldn't have been an issue for the older modem?