Can anyone confirm this? Did you try with just the Fishing+ mod enabled?
For me the options show up fine in both, Solo and Host mode.

I've had the same issue as well. Adding the mod doesn't add the sandbox vars to my dedicated server.

I tried adding the lines manually, but the settings go away on restart.

There is an easy solution to this problem. I was stuck with a problem like this for other mods, because it's so strange. You have to have the mod enabled in the 'Mods' tab on the title screen. Then, turn on the mod in the server you want to host. After that, you should get the option to change the sandbox settings of the mod. You're welcome!

Thank you very much for clarifying!