ほんとにメモだけになりますが、公式でも下記をやるようにとでてたので、下記は必須で確認が必要です。
Synology NAS のセキュリティを強化する方法
https://www.synology.com/ja-jp/knowledgebase/DSM/tutorial/Management/How_to_add_extra_security_to_your_Synology_NAS
synology ds218+のセキュリティ対策
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