Invitations to online friendlies in FIFA 19 on Windows 10 are issued and received correctly, however after accepting (pressing Y for Yes), the game displays a blank screen with a theme background and then disconnects with the message "lost connection." The game works perfectly when I use my Airtel Mobile Hotspot 4G, and I can play with my pal. The router firewall was disabled by an Airtel expert, but the problem still exists.
I think rather than disabling the firewall altogether you instead need to open ports in the firewall to allow the data from FIFA 19 to transfer through. I had a little browse online and found this article which details the ports that are required to be open for FIFA 19 to work correctly: Forwarding Ports in Your Router for FIFA 19 (portforward.com)
The process for opening ports will differ depending on what router you are using. You can find guides online for almost any router. Assuming you are using a BT router, and assuming that it's the most common router, their SmartHub, then you can find a guide on how to open ports within the User Guide here: BT Business Smart Hub | User Guide | BT Business | BT Business (custhelp.com)
There are also guides for BTs older routers (e.g. the Hub 5 or Hub 3) on the BT website, which you can find easily enough 🙂
Hope this is of some help! 😁