Go to the BP_ShadowDummy and create a new function called SpawnAttachment then add a new function input called Attachment and set its type to BP_MasterAttachment class reference (the purple option) and add another input called SpawnTransform and set its type to Transform.

Add the highlighted nodes shown in the image below, location at the top of the image.

Then create another function called RemoveAttachment and add an function input called Attachment and set its type to BP_MasterAttachment.

Add the highlighted nodes shown in the image below, location at the top of the image.

Then create a new variable in the BP_ShadowDummy called Attachments and set its type to Actor and set it to an array.

Add the highlighted nodes shown in the image below, location at the top of the image.

Go to the BP_MasterWeapon then go to the Attachments collapsed node and create a new event called ClientRemoveAttachment, set it to Run on Owning Client and add an input called Attachment and set its type to BP_MasterAttachment.

Add the highlighted nodes shown in the image below, location at the top of the image.

Add the highlighted nodes shown in the image below, location at the top of the image.

Add the highlighted nodes shown in the image below, location at the top of the image.

Lastly you need to go to each attachment blueprint and copy the video below.