I have had a CAX80 (Firmware Version V2.1.5.2) for just over a week, and have experienced the same issues with uploading on the 5GHz WiFi. In my case, I found I could not send an email message with an attachment larger than about 200KB. I confirmed that this was the case whether I used Outlook, webmail or a browser extension, and regardless of the email address or domain.
Since I have access to the servers I use for my business, I was able to track the failed email transactions in the postfix (mail transfer agent) log. Here is a typical example:
...[DOMAIN REDACTED] warning: TLS library problem: error:1408F119:SSL routines:ssl3_get_record:decryption failed or bad record mac:ssl/record/ssl3_record.c:677:
...[DOMAIN REDACTED] postfix/smtpd[284252]: lost connection after DATA (188379 bytes) from c-XXXXXXXXXXXX.hsd1.co.comcast.net
The above errors seemed to indicate a timeout issue and misconfiguration of SSL/TLS such that the sending server wasn't able to negotiate TLS / agree upon a cipher. Since this mail server has not exhibited any problem behaviors at all, it was puzzling.
After spending a couple of hours with Netgear support, which went nowhere, I continued troubleshooting on my own, and discovered that the problem disappeared if I used a VPN (I have Mozilla VPN, and routinely turn it off for various operations that don't permit host obfuscation).
I have been testing this for about five days now, and so far the results are completely reproducible. I can send emails with large attachments over the 5GHz radio using the VPN, but not without it. Using the VPN does, of course, result in a loss of maximum download speed - in my case download is slowed from 1000Mbps to about 700Mbps. Interestingly, upload is unchanged, at 23Mbps.
While still only a workaround, switching on a VPN is a lot easier and less disruptive than switching to the 2.4GHz network to upload a file or send an email with an attachment (which is what one of the Netgear support people actually suggested!).
While we all wait for a real fix from Netgear, you might want to try the VPN workaround. I'd be interested to hear if anyone else can corroborate.
