So, I have a RaspberryPi3 b+ with Raspbian 4.14.78 and a Ubuntu 16.04 laptop. Now I want to use the laptop as a server and the RaspB as a client and have them communicate wirelessly. But I would prefer to do it locally, so there is no need for an internet connection. I have created an Access Point from the raspberry following this guide. Now, I can connect to it with my Win laptop but not with my Linux. In the Linux machine I can see the network, try to connect to it but after 1 min tops it gets disconnected. I have no Ethernet connection on the raspberry, could this be the problem? Another problem is that from the windows PC I can not visit the page served by the raspberry on 0.0.0.0:8080. Finally, now, on every bootup the raspberry has the AccessPoint activated by default so I cannot connect to other networks that provide communication with the world outside (internet access). How can I activate and deactivate the access point I create when needed?? Thank you in advance for your help.
It is very difficult to say why the connection from the Ubuntu laptop disconnects after a short time and why MS Windows cannot view a page 0.0.0.0:8080 from the RasPi because lack of detailed information about your setup. It implies that there is a web server running on the RasPi but you don't mention it. You say you want to use the RasPi as a client so the Ubuntu laptop should spawn an access point. Then you say you don't need an internet connection but some sentences later you write you want to connect to the internet. But I will try to give some general information that may help you.
For clarification I assume that the Raspberry Pi acts as a server and spawns a wifi access point. A MS Windows and Ubuntu Laptop connect to it as stations. On the RasPi is also a web server running that will serve pages to the stations. To connect to the internet from time to time you can manually switch the access point to a station and connect to another access point with internet access and if done manually switch back. How to do it you can look at Swapping between wifi client and access point?.
You can also make an access point together with a client connection to the remote internet access point. Then you do not have to swap the connection manually and established connections from stations are not interrupted. How to do it you can look at Access point as WiFi repeater, optional with bridge. If you omit setting
/etc/systemd/network/08-wlan0.network in that tutorial then you also have an isolated local access point. Stations cannot connect to the internet.