Freeing up filesystem space

Evening all. Running Reboot (17.01.4, r3560-79f57e422d) on a Linksys WRT1900ACS and it is running nicely, except that I have no free space, and I am confused as to where it all went. I want to install a couple of programs (transmission), but I can't, as I get an error about insufficient space when I try. Running DF-H shows this:

Filesystem                Size      Used Available Use% Mounted on
/dev/root                 2.8M      2.8M         0 100% /rom
tmpfs                   250.8M      1.4M    249.4M   1% /tmp
/dev/ubi0_1              23.5M     23.0M         0 100% /overlay
overlayfs:/overlay       23.5M     23.0M         0 100% /
ubi1:syscfg              29.6M    444.0K     27.6M   2% /tmp/syscfg
tmpfs                   512.0K         0    512.0K   0% /dev

So, where has all my free space gone? I've only installed a few basic things (Samba server, etc), but all my free space seems to have gone. Any tips on clearing out some space, or figuring out what the issue is?

That's a good question. According to the link below it should have 128 MB NAND flash. But you seem to use less than 32 MB (rom 2.8 MB, overlay 23 MB, kernel x MB).

@mikma The 1900ACS has multiple partitions, including a backup partition, all of which is detailed under Flash Storage Layout, with a total of 40MB per primary and backup [alternate] partitions, split into two sections, 6MB for the kernel, 34MB for the firmware.

@richardtheb It looks like you're booted to the partition running Linksys OEM firmware, not the partition containing LEDE.