The Gen3 Sub features the latest radio so it might be marginally better, but I wouldn’t bet the purchase price of a new Sub on it.

Your fundamental problem is that the Sub -- or any other home theatre satellite -- was not designed to be used in a different room from the master player. It connects to the master -- the Amp in your case -- by a local, direct 5GHz link. This clearly suffers from attenuation if there are walls and floors in between. 

Presumably the Amps are wired to the network. The best option would therefore be to run a wired connection to the Sub as well. 

I’m sure the new Sub v3 has better WiFi adapters, but there is still the ‘possibility’ it might not resolve your issue as the Amp needs to link to the Sub over an ad-hoc 5Ghz connection, that band has the speed required for keeping the TV audio in lip-sync but it still may not have the reach/penetration that you require for your particular setup. I personally think you would more-likely be better off either bringing the Amp and Sub closer together, or resort to wiring ALL products in the setup.

That’s not to say you couldn’t try it, of course, but I suspect it may prove to be no better than what you have already.

Ah @ratty beat me to it - but I was thinking along the same lines too, it seems.

Fully aware of the limitations. 
 

Initially, I was able to force the Sub to connect via the 2.4Ghz by disabling WiFi on the Amps. However, after a blackout I lost that connection and haven’t been able to get it back. 
 

I might be able to get an Ethernet cable to the Sub. Should it solve it if both the amps and the sub is wired to the router or is it really only intended to work with 5Ghz?

Wiring both the Amp and the Sub would bypass the wireless connection, so should work fine. 

Forcing the Sub onto 2.4GHz by disabling the Amp’s radios would have been an imperfect solution, since latency around the 2.4GHz route could have been pretty variable. In previous discussions I’ve suggested it might work, but it was definitely a case of YMMV.

And it’s fine wiring to the router, right?

Not wiring the sub to the Amp. 

It shouldn’t matter, assuming the router’s LAN ports are simply switching between one another (i.e. the data path doesn’t involve the router software).

Wired the entire setup and everything is working perfectly. I am using a mesh-setup and was able to move one of the transmitters close to the Sub and hardwire the sub to the Ethernet port on the transmitter. The two amps are connected directly to the router. Works great. 

See how it goes then. If you’re relying on a WiFi mesh wireless backhaul to interconnect the Amp and Sub you could still see intermittent latency spikes which may cause the Sub to drop out.