My TP-Link AV600 works fine downstairs, but does not work well upstairs:

• Shows as fully connected but there is actually no internet connection (e.g. items such as email and MS Teams show as offline; website pages do not open)
• Very poor and slow connection
• Shows as limited and does not connect to the internet

I am at a loss as what the issue is. I have attempted to connect it in different rooms but the issues persist. 

Would appreciate any advice.