What Duplicati version? Newer Experimental and Canary do parallel uploads to try to make them faster:
C:\ProgramData\Duplicati\duplicati-220.127.116.11_canary_2019-09-05>Duplicati.CommandLine.exe help asynchronous-concurrent-upload-limit
--asynchronous-concurrent-upload-limit (Integer): The number of concurrent
When performing asynchronous uploads, the maximum number of concurrent
uploads allowed. Set to zero to disable the limit.
* default value: 4
You could try lowering that (if applicable), or try raising concurrency on iperf3, for example with -P option:
iPerf 3 user documentation
-P, --parallel n The number of simultaneous connections to make to the server. Default is 1.
Some scp/sftp programs also do concurent uploads. It wasn’t clear what your scp upload test was using:
Connections (Cyberduck program)
You can choose to use a single or multiple connections for file transfers.
Non Backend Flags (rclone program)
–transfers int Number of file transfers to run in parallel. (default 4)
Are you running with default 50 MB remote volume size, and with encryption (i.e. not ordinary .zip files)? Although I can’t find the evidence, ASUS Trend Micro deep packet inspection initially made me wonder if content scanning was going on in real time. It looks more like not, but details of DPI are extremely few…
Basically, if you have enhanced security (including ASUS Trend Micro AiProtection), try testing without it.
–throttle-upload or the throttle button near the top of the GUI page can try to limit upload speed, although because of bugs it sometimes limits download speed to the same level. Other bugs corrupt data, so you should use the latest canary if you want to try low speed throttles to see if you can keep your gear going.
Many routers also have throttle controls of their own, and at least mine seems to do a better job than the throttle Duplicati runs (which can get kind of bursty, and possibly high peak bursts are a factor in failure).
Watching the interface transfer rate in Windows Task Manager or whatever you have can show behavior.
When the connection fails, how does it fail? Is it Wi-Fi disconnects? Is reconnect to the router possible? How do connections from other devices (including phones and tablets on Wi-Fi) drop at the same time?
When Duplicati detects a drop via a break in the upload, it retries up to –number-of-retries times, and the attempts and error messages should be seen in server live log at About --> Show logs --> Live --> Retry however if you prefer –log-file at –log-file-log-level=Retry, that will do the job without you watching over it.
Does your ASUS software keep logs that might help? Some modems do. I know my cable modem does.