Please upgrade OpenVPN/OpenSSL version
I see there was a thread opened about 1 year ago to asking for an upgrade of the OpenVPN server on the ER605
Please upgrade OpenVPN/OpenSSL version - Business Community (tp-link.com)
The ER7206 has the same problem and also needs its OpenVPN/OpenSSL versions updated.
The newest Windows OpenVPN GUI (downloaded today) doesn't allow the "--data-cipher" option in the config file, so that's a show-stopper.
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There was a post that suggested a "workaround" is to try to find an old version of the OpenVPN GUI for Windows to get it to work.
But, I don't know if I could find an antique version of an OpenVPN client for iOS/MacOS or Android.