I bought the 4K HDMI Extension Adapter and connected it to my Samsung plasma tv, but the image is not HD. Before this connector was secured, the image shown by my TV was in HD.
What’s going on with this situation? 
During the transmission of the signal, passing through an HDMI extender can cause signal loss, resulting in a decrease in picture quality and an inability to reach the highest level.
It may be that the total length of the HDMI cable you connected exceeds the range that the HDMI adapter can achieve 4K performance.
Please ensure that your connection is secure and tight. You can also check the settings on your tv to select the appropriate resolution. Lastly, you may consider using an 8K HDMI Adapter if your Samsung plasma tv is also 8K compatible.