How long is it out when it drops? Have you worked with your ISP, ruled out local Wi-Fi glitches, etc.?
When it doesn’t drop completely, does it perform well on upload test (may vary by test), or is it slow?
Measuring network performance: links between latency, throughput and packet loss explains factors impacting performance. If flaky means some packets get lost, connection speed will suffer from that. Retransmission attempts will be done, but these take time. Duplicati can retry too. Again, takes time.
--number-of-retries = 5
If an upload or download fails, Duplicati will retry a number of times before failing. Use this to handle unstable network connections better.
--retry-delay = 10s
After a failed transmission, Duplicati will wait a short period before attempting again. This is useful if the network drops out occasionally during transmissions.
You can set those as high as you can stand to wait. Sometimes it may be better to give up and bail out.
Please look at the destination files. By default you should have a bunch of 50 MB dblock files, each with dindex file that indexes it. While some might be truncated (if network died in middle), good ones persist.
Any Duplicati backup needs to look through all the files to see what changed, but uploaded data should not upload again. In this sense, next backup is automatically a resume. It should even upload a Restore entry (please look) for the interrupted backup to get what was done so far. Typically this is the previous backup plus what was backed up before interruption. It possibly differs if initial backup has not finished.
After interrupted backup, Duplicati wants to start re-uploading everything! was a similar retracted issue.
EDIT:
Although poor network performance might be hiding it now, Duplicati performance slows when tracking more than a few million blocks (database operations slow). For 2 TB you’d want blocksize of 1 or 2 MB instead of the default 100 KB. Unfortunately you can’t increase blocksize without taking a fresh backup.
Depending on how fast and stable your network is (or isn’t), is cloud backup/restore even a good idea?
Maybe your download reliability is better? On some network technologies (e.g. cable), that can happen.