I am having the same problem. My phone is LG V20, running Oreo. I have tried the following FTP servers:
Remote File Transfer LIte
FTP Server Ultimate
SwiFTP
Primitive Ftpd
FTP Server
..and probably some others that I deleted after findint they didn't work.
On a previous pone I used SwiFTP successfully, and on my tablet (currently running Nougat, but it wants to update to Oreo - haven't let it yet), another app called FTP Server work. On the LG V20, however, I set it up just as on my tablet & previous phone (but giving it the correct location for my sd card). I start the server on the phone & it gives me an IP address (sometimes more than one). I use my FTP client on my computer (FileZilla) and enter the login & password set on the phone's server. The client continuously fails to connect. The only error messages I get are from the client, which is just a failure-to-connect message.
I'm guessing this is an Oreo thing since I didn't have trouble on other Android versions, and so I'd like to get it resolved before letting my tablet update itself. I have found that USB transfer is hit-or-miss, and FileZill works great on the tablet and previous phones.
I've also tried to connect using my browser:
ftp://[ipadress].[port], and that works fine on the tablet, fails to connect on the phone.