I now tested my wired Apple TV 4K and it is getting the performance I expected. 308 Mbps down and 345 Mbps up. So it seems it’s the wireless network that is performing poorly which is very disappointing. Do I return this WiFi router or is there a way to fix it? I got this in the hope it would improve the WiFi, not make it substantially worse

  @Crunchewy Check to see if you have QOS on.  I found my speeds are better with if off.

  @Crunchewy A long time ago ISPs used to give trouble to non-ISP provided routers that I suspect still might be going on today.  Maybe try cloning the MAC of the prior router onto the new one and see if speed improves.

  @jzchen interesting idea, but if I do that will they still think I'm using their router and then charge me the rental fee? I guess as long as I wait until they get their router back maybe it won't be an issue. 
 

And actually as it turns out, in practice this router is still giving better results. For example PS5 remote play is working great in my house, unlike with the Verizon router. So as it turns out it's not actually worse, only in upload speed. I think I can live with that. But I will try that MAC cloning idea after I get theirs returned. 

I did try copying the MAC address of the Verizon router, but it had no impact. In retrospect that wasn't likely because the wired internet performance is as it used to be. It's only the wireless upload that is "worse", which is all on the router. But I am not going to worry about the speed test upload speed because in practice actual devices work better than before. The most obvious difference is PlayStation remote play which was poor in the house on the Verizon router, and works great on this tp-link. So I'm a happy camper as that was one of the main things I was hoping to improve.