ReadyCloud service is actually available for OS 6 FW only. Unfortunately if you have the Duo which runs on OS 4.x or the Duo V2 that runs on OS 5.3 then ReadyCloud will not be available for it. there should still be apps available for your Legacy NAS. You may want to check on apps.readynas.com or https://rnxtras.com/ for apps.
If you don't have OS 5.3 on your Duo then that is a V1. You cannot upgrade the Duo to OS5 or 6 unfortunately. You may want to try using VPN connection instead.
Please check on this article and page 99 on the manual regarding installation of Addons. Alternatively you should be able to install apps manually from your NAS as well.
Installing the apps requires Admin access, remote access will be possible if you have VPN Connection setup since ReadyCloud is not available.
ReadyNAS Remote, which was available for legacy units like yours, has long been discontinued. ReadyCloud is only for current units and specific older ones that can be updated to OS6. Yours has the wrong processor type for that update.
One option is to set up a Virtual Private Network (VPN). Many routers support that to give you remote access to your entire LAN, and that's typically the easiest way to do it, especially for a novice. There are ways to run a VPN on the NAS itself, and pointers to those methods have already been posted. Something like a Raspberry Pi can also be used, but that's even more complicated.
Another option is to use something like LogMeIn, RemotePC, GoToMyPC, TeamViewer, or Chrome Remote to remotely access a PC on the same LAN as the NAS and then do "local" admin via that computer accessed remotely.
To be honest, if you want it all within your NAS, your best option to get remote access to your NAS is to buy a current model instead of a 5+ year old out-of-date one. One that will run ReadyCloud.
