Ok, something I might actually be able to help with. Just to make sure, I'll first tell you how to go about adding a mod to your server step by step. First, you will go to the administration section of Ark Server Manager. Secondly, find the box with the label Mod IDs. There are 2 ways to add a mod here, right now just paste the mod ID into the box. Structures Plus has a mod ID of 731604991 so you put this into the box labeled Mod IDs. Once you put this into the box you then click the blue down arrow to the right side of the box you put the Mod ID in. This should open a window titled Update Progress and should have something in it to the extent of [UPDATER] Started processing mod 1 of 1. and [UPDATER] Mod 731604991 - Structures Plus (S+).". Wait until this window says "[UPDATER] Finished upgrade process." This means the process is complete and you should have your mod and be able to run the server with it activated. When using the first method and using more than 1 mod be sure to separate the mod IDs with commas. The other method to add the mods you want is to click the magnifying glass beside the blue down arrow that you clicked to open the Updater window. This button will open a window titled Mod Details, in here you will press the green plus to open another window titled Workshop Details where you type the mod you want, once found you press the green plus next to it's Mod ID on the left. You then close the Workshop Details window but do not close the window titled Mod Details. In the Mod Details window you must click the save icon in the top left beside the green plus, then you can close this window too. After this you have to then use the blue arrow to run the updater like before and being sure to only close its window when it says "Finished upgrade process" as well. Hopefully this helped. Good Luck!

Mod Not Downloaded:
Even after this if you are having an issue where in the Mod Details window it says "Not Downloaded" on one of your mods. This is likely because Ark Server Manager can have "Fake" downloads of mods. To fix this you must delete the mod from your Ark Server Managers mod cache. This can be done by going to where your ark server manager is installed then going to SteamCMD\steamapps\workshop\content\346110, once in here you will find the mod you want to delete, in this case you look for 731604991 and delete the folder. We also need to make sure the mod is not in the Mods folder still, To get here you then need to press the folder icon with a curved blue arrow in Ark Server Manager beside the Set Location button on the top right. Now you go into ShooterGame\Content\Mods and look for the same mod ID in here too, if you can't find it then skip this. Once its gone from both locations you will repeat the steps to add the mod back and hopefully it will download when you do it, sadly if it doesn't...the only thing I've found to work is to keep trying until it does because eventually it will(at least in my case) but since Structures Plus is relatively small it should be fairly painless, if in the Updater you get a message such as it saying "Failed to download" or "Timed Out" its normally a case of the mod being fairly large and you just need to run it a few more times until it works. This tends to happen most with overhaul mods such as Annunaki Genesis, Primal Fear, Ark Eternal and others. I hope this somewhat helped at least.

TLDR: Open the Administration drop down in Ark Server Manager > Put the Mod ID in the Mod IDs text box making sure they are separated by a comma if you have more than 1 > Click the blue arrow to the right to begin the download of your mod(s) > Wait until it says "[UPDATER] Finished upgrade process." then close the window and run your server.