You have two options that I can think of immediately:
Option A (WARNING THIS WILL BE EXTREMELY SLOW)
Keep the tables separate as they are now and write a DAX measure using SWITCH to calculate which web connection to sum
Option B (recommended option)
Append all the web connections in Power Query:
1) Open Power Query
2) Go to the Germany web connections table
3) Add Column > Custom > = "Germany"
4) Repeate this for each country
5) Append Queries as New > 3 or more Queries > select all the web connections
6) Relate the 'List' from your slicer to the new custom country column in your append query
7) Use the Apped query data in your line chart.