@pkz78 Some things I would try...

On the router setings try different channels and band width.

Power down any extenders, boosters, if any in your system.

On the iPad, delete (forget) the network and then re-add it.

Other steps you can take on the iPad https://appletoolbox.com/ipad-air-2-wi-fi-problems-fix/

Let us know if something works for you.

@ArcherC8 Thanks for the suggestions. I've tried everything. Same setup as old router, even disabled all signals like bluetooth, disconnected all devices, forget all settings, reset cache in android, fully restart all systems, android literally near the router etc., - still same result for both ipad and android - download speeds never exceed 40mbps (in best case scenario) for 5GHz on a 200mbps ISP line...

I've also tried all combinations of band width and channel there is in the tplink settings page. All of them yielded more or less the same result. In none of the tests did I see normal download speed. 

@pkz78 Creating another post for clarity (sorry for double post).

So, I tried another experiment. I hooked up the line form my ISP to the WAN of my tenda and connected one of the available ports to the WAN of Archer C7.

Now, I tried testing wifi 5g from archer and I'm getting slightly better speeds. Why could that be? how can the repeater performance be better than getting direct connection to WAN?

Edit 1: another update: Tried setting up my tp-link as a fresh router after factory reset and connected via PPPoE instead of simple dynamic ip (not sure if they are the same, but I remember not having to input my ISP username and password when I set it up the first time). Now, the speeds are slightly better on average. BUT, the deviation is very large! I sometimes get 5mbps up/down and sometimes even 100+ mbps up and down. I'm testing this with 2 android phones both with wifi ac, 1 iPad and 1 macbook. Been running around my home all day today trying to get to the bottom of this.. I've tried everything. It still seems like my old tenda router is far more stable and much faster on average compared to tp-link. Open to any other suggestions!

Edit 2:another update: I'm frequently seeing (now and before reset) my speeds start higher but then get continuously lower and lower when doing speed test. This again, does not happen with my other router. and yes, I have tried all band channel combinations.

@pkz78 I am assuming durring this time the wired connections still perform at the rated speeds. Next I would try changing the 5GHz SSID and password and connect only one device at a time and test to ensure you do not have another device causing some issue. Since you have a MACBook, connect it via WIFI and then while holding the option key down, click on the WIFI symbol at the top of your screen. Then click on the Wireless Dianostics, it will open a page but do NOT click CONTINUE, instead click on WINDOW in the upper left hand side of the screen, then you have several windows options where more info about WIFI can be found. SCAN is good to see other WIFIs and channels used. Performance is good to see if your signal strong and is consistant. Info gives info about the WIFI and look at Quality as it should be Excellent. It also shows if you are connected both IPv4 and IPv6. There are other window options that you can explore also.

@ArcherC8 Thanks for giving more suggestions.

I'm not near my PC atm so I will let you know the results when I get home.

How can changing SSID help solve this problem? If I understand correctly, SSID is just a unique identifier for wifi signal right? Please do correct me if I'm wrong or if I have misunderstood something.

@ArcherC8 

Thanks for the explanation. I just tried another round of experiments. Disconnected wifi from all devices, changed SSID, changed password - connected my phone and tried speed tests again. I still get the same result. There were some runs where I got >70mbps on 5GHz, but that was an outlier. Among 10-15 different runs (1-2min gap in between), the avg speed was around 40-50mbps only. I then connected my iPad and other devices - it was still the same. So, I guess the reason for slow down is something else :( 

I've asked for a refund for this (amazon) as I'm out of options. I still have ~ 1 day left. If I can get it to work within that time, I will keep it, but from the results I've been getting in my tests, they are not looking good. :( 

@Kevin_Z 

I spoke with a customer rep. from amazon yesterday night. They told me to go ahead with the refund because the refund window (1 week) is closing today. So, I agreed to it.
But if some suggestion makes everything work well, then I'm fine with canceling the refund and keeping the product. Unfortunately, since I was traveling a lot this week, I was not able to test all these as soon as it got delivered. So, I've basically been running speed tests and signal tests all day long since yesterday. I've also tried all the suggestions posted here. If there are still suggestions/possible solutions I've yet to try, I will try them and report back immediately since I still have about ~24 hrs till I return the product.