slower than expected readyshare upload speeds

It may be worth going back to the most basic aspect of the issue. You stated that the backups to the USB storage connected to the router was just as slow as the WAN connection.

If that is the case, then a better test to do first, is to use windows file explorer and type in \\your router's IP address

For example, my RAXE500 is using 192.168.1.1, thus to access the USB storage, I would use \\192.168.1.1

After connecting to it, attempt to copy a single large file to the storage (a file that is 1+GB should be fine).

Examine how fast the speeds go.

The RAXE500 using a USB 3 drive formatted as NTFS, will read and write at 145-147MB/s when you are connected to the 2.5GbE port, and fully saturate a gigabit Ethernet connection for reads and writes when using a 1GbE port (at around 115-117MB/s).

Different partition types can cause issues, as well as misaligned partition table (improper sector alignment), as that can significantly increase the number of IO operations needed to complete a transfer.

All consumer routers struggle with tasks requiring a high IOPS, as smb is not well suited for such tasks, that is why when you copy a large number of tiny files, it will slow to around 1MB/s or less, where the same transfer will be faster on a slower device such as a raspberry pi doing a file copy from 1 USB drive to another on the same device.

Some backup software can also cause slow speeds depending on how they manage manage backups. for example, if it constantly updates a log file on the backup location while also individually copying tiny files, then it will be even slower than normal.

If the fie transfer using windows explorer is fast while the backup software is slow, then it would mean that backup software needs to be reconfigured to better work with the smb share.

As for speeds changing when you disconnect the modem, are you using any VPN software? also are you using and QOS software on your PC, Some systems as well as some motherboards will come with rather annoying software packages that will include QOS software that can lead to issues with network shares. Some VPN applications can also cause weirdness with local connections, especially if the split tunnel is buggy.