How can I force Android to connect to the network of current country?

I live in country A and work in country B. I have a mobile phone contract from country A. A and B are both in the EU so there are no roaming fees at all. During the day I have no network problem, the phone uses the network of the country it's currently in and everything is fine. Problems arises when I cross the border: it seems to me that the phone tries to stay connected to the network it was previously connected to. So for 10 minutes or so after I cross the border, the network signal is weaker and weaker until previous country is too far away. How can I force the phone to connect to a country's mobile phone network as soon as it enters in said country? NB: situation is exactly the same when traveling from A to B or from B to A. What I tried: switching to plane mode just after crossing the border, waiting 5 seconds and switching back. It doesn't work.