@David-TP , thank you for picking this up. The modem is a Zyxel T-50 (T-Mobile). The issues are similar in both router and access point mode..

  @awdhaan and @David-TP 

I'm having the exact same issues on my recently installed BE85 w/ 3 nodes.

General browsing, streaming, calls, etc., work fine, but any "large" downloads fail at random points. It can sometimes fail starting the download, other times halfway through, or somewhere in between.

My setup is somewhat similar to yours, the Deco is set to Wireless Access Point (it does NOT handle routing), and the main Deco is connected to a separate router (non-WiFi) for my internal network. I am, however,  not using ethernet backhaul, I'm using wi-fi backhaul.

Here's what I've tried so far:

- Tested multiple devices, including Macbooks, iPhones, and Windows PCs, with the same result. As you pointed out, iOS updates are a no-go as they fail to download, same with downloading a relatively large (1+ GB file).

- Tested regular and dedicated WiFi backhaul, same thing.

- Disabled bean forming. Same result.

- All devices, regardless of the WiFi band experience the same problem. Whether it's WiFi 5, 5, or 6E enabled devices, the same issue happens.

- Rebooted and factory-reset my Deco a couple of times. Upgraded to the latest firmware available on the TP-Link site.

- If I connect a device to the ethernet port of any of the Decos (even the non-main ones), things work as expected. This means that deco-to-deco wireless communication/backhaul works just fine, and it only affects wireless clients.

- Tried disabling the 6Ghz network, using MLO, etc. Same issue with any of those combinations.

- One thing I did notice is if the download is slow (i.e. transfer rate of around < 50Mbps), it does not appear to fail. But when it gets to around 100+Mbps, that's where I generally see the download failing. This is not entirely conclusive, but something I've noticed. 

While I expect issues to be worked out on new devices, it's pretty ridiculous I cannot download files. I got the Deco BE85 to replace my 6-year old Orbi mesh, and I'm regretting that decision, never had these issues with Netgear.

  @FabianC I've returned all the devices and got a full refund after extensive troubleshooting. TP-Link provided great support, but we were not able to resolve the issue.
I now implemented the network using another brand. No more download failures. I'm happy now.

  @awdhaan thank you for the response

That's a shame, I'm really hoping it doesn't have to get to that. Just curious, which mesh did you go with?

My runner up is the Orbi 960 if TP Link cannot resolve it. 

  @FabianC I implemented a new network using Ubiquity Unfi: A Dream Router, a couple of PoE switches and 3 additional PoE AP's. Works extremely stable and great telemetry. The only thing I still need to figure out is some form of parental control, but there seems to be options for that.

Just to keep this thread updated for those having the same issue.

- TP-Link has provided a beta firmware to test, but it did not resolve the issue.

- I have also tried disabling beam forming, same issue.

I have 5 days left until my return period expires, really hope I get some sort of resolutions soon @David-TP 

Just a quick update...while the issue is not yet permanently fixed, the engineers working on the issue have provided a temporary workaround which addresses the problem.

They have collected information from my setup (through a remote session) and are working on the actual fix.

@David-TP a huge thank you to you for getting this moving and to  Aaron on the engineering team, they have have been working on this diligently and the communication with them has been great.