do a manual network search and select a provider.

I tried this on available networks manually but doesn't connect.

You might need to try several times - it can take up to 10 attempts depending on the selected provider.

Dear! Should I attempt 10 times on same network or different. Because there are more than 7 different networks. I tried all but Neither connected. Moreover tell me . The roaming service should be activated on o2 Sim also. Or by default it is already activated?

You should attempt it on the same network. Roaming in France should be possible with Bouygues Telecom, Orange France, Free Mobile and SFR. If none of these work your SIM may have a roaming block that would need to be removed by customer services.

How I have to contact customer service. Do you have email address or other source so that I contact with them for unblocking roaming 

Try the chat in the morning: o2.de/chat

Ok thanks dear

Hello  ​@Zeerosry 
Welcome to our community 
It's great that you've contacted us here and that you've already received such great support.

Have you chatted with us? Was a roaming block found on your SIM card and removed?
Best regards

Kathi

No dear I don't know the roaming is blocked or not on my sim card. But roaming service does not work. I tried many times to connect various Networks shown to me in sainte-menehould but in vain. How can I check whether roaming is allowed for my sim card or not

https://www.o2.de/kundenservice-chat