All of the primary consumer products competition use WiFi as well, so you are likely to have the same problems with those, unfortunately. That includes remotesy. They all use Wi-Fi to allow SmartThings to send them a signal, then they can send an infrared signal to the AV equipment.
There are alternatives that are ethernet based, although they are more commonly used for offices and commercial buildings.
But That just gets the signals to the television, you still need some kind of server device to Control what signals to send when. And something to integrate it to SmartThings. And none of that is going to give you a button remote that you can hold in your hand as well.
So I would suggest dividing it into two problems.
If you just want a universal handheld remote, go ahead and get one that doesn’t use Wi-Fi but just depends on direct IR signals to the devices in the room. There are lots of these.
Then you can consider whether you want to go ahead and put in a server based system to allow for SmartThings control of your home theater equipment.
One option would be to get the global cache unit which uses ethernet rather than Wi-Fi, and then add hambridge running on a mac mini to run the integration between smartthings and the global cache.
Setup is certainly going to be much more complex than with Harmony, but you would end up with the ability to do a full integration with SmartThings. The only thing is that the button remote in this scenario can’t initiate anything in SmartThings, you would be running to parallel systems. The handheld remote for use in the room and the server/global cache for integration with SmartThings and One of the voice assistants as well if you want that (which would be going through SmartThings).
If it was me, I would probably bring somebody in to figure out why the Harmony isn’t working. Maybe you could move the hub Close to an access point and run some longer IR emitter cables. Maybe you need a different router or just a different Wi-Fi channel. But given the number of home entertainment devices now that rely on Wi-Fi, personally I would like to get a good Wi-Fi signal near my television. But that’s just me.
But if you want a real alternative that doesn’t use Wi-Fi, the global cache ethernet-based equipment is excellent and widely used, but again the technical set up is much more complex.