Why Need Upgrade the OpenVPN Version
In Yeastar U-Series and VoIP Gateways, the OpenVPN version is 2.0.5. If the OpenVPN server uses TUN mode / Subnet Topology, then the Yeastar client side can't connect it unless upgrade the version to the latest one 2.4.3.
For Gateways (except for TGv3/TAv3)
How to Upgrade the OpenVPN Version
STEP 1. Follow this guide to login the device via FTP: https://support.yeastar.com/hc/en-us/articles/217380628-How-to-Access-MyPBX-FTP-Folder.
or the persistent folder contains the "imageupdate" sub-folder which would be like this:
STEP 3. Reboot the device to make it take effect.
Please try to upgrade the firmware to this version: http://download.yeastar.com/YeastarSupport/image/126.96.36.199.4.bin
please download the attachment openvpn246.tar, and upload it to the App Center to upgrade the OpenVPN version.