Could I use the present perfect and the past simple as in "Since there's been no internet access,I couldn't get online." ?
Present Perfect tells us that there is still no access to the internet? If not, what else does it tell us?
I think it is possible to use past simple instead of present perfect but then the sentence would meant that there was no access to the internet in the past. Present Perfect suggests that there is still no access, this tense has its reference to the present.
As I wrote above, I believe that there is still no internet access, but I'm not exactly sure about it.
I believe that "since" states the reason because after that there is the result = "I couldn't get online".
Good question ostap77.
"Since I've been in France,I could speak French fluently." = Present Perfect is also used to express the things that we experienced (like visiting France, as you wrote). It doesn't mean that this person is still in France.
I'm also waiting for the teachers' intervention to clear this up.