What custom firmware are you using?
I installed DD-WRT’s 30826 version of factory-to-ddwrt.bin for the TP-Link TL-WR841N (http://dd-wrt.com/site/support/other-downloads?path=betas%2F2016%2F11-01-2016-r30826%2Ftplink_tl-wr841ndv11%2F) on a brand-spanking new v11.1 just yesterday and had no issues at all getting back into the router’s web interface. Note that it did change the IP range.
I’ve even got the Hotspotsystem integration working and am in the process of setting up the splash page.
This is sooooo much further than I got with OpenWRT’s firmware where, like you, I lost WebGUI access because the ROM was only 4MB and couldn’t install the LUCI package that was part of the OpenWRT bundle. I bricked that router purposely after that and sent it back to the supplier as DOA, got the new router and dived straight in with the DD-WRT I mentioned above.
Given that all of your TP-Links are probably out of DOA period, you’re going to have to use WinSCP and try to revert back to the TP-Link original firmware and then do the DD-WRT upgrade like I did.
Or you could see if you can do an OpenWRT-to-DDWRT firmware replacement…
Best of luck!
Ps. another win for the DD-WRT firmware is that the Hotspotsystems integration is built in and just needs to be enabled and configured. Unlike OpenWRT’s setup where you have to generate a script on the HSS website and then install that onto the OpenWRT router.