finding the source of dropped frames is not so easy and expensive if you don't have the equipment. here at work i just get another switch with port mirroring functionality from IT and capture traffic then compare. it can be very tedious though.
the web uses HTTP and that is typically over TCP, not UDP. dropping a single TCP frame is not a big deal and TCP will re-transmit if needed.
if the phones and tablets work fine over wireless then that is a clue, it tells me that the modem is very likely OK as well as *some* part of your router. try to get your zwift machine to use the same wireless network. some wireless routers are multi band so make sure you are on the same band. it is also possible that the dropouts are caused by your wireless adapter in your zwift machine. you can try a real Ethernet cable if practical.