The multilayer game mode over android is for devices connected on the same WiFi connection. We currently have no known issues with multilayer connectivity issues in MMR. Mini motor racing's networked game play relies on an existing stable network, if devices on a network cannot communicate with one another (for instance due to not having been assigned IP addresses by the DHCP server, firewalls blocking communication between devices or by being on different subnets) the networked modes will not work.
Some good things to check:
- Are you able to test it on a different WiFi connection?
- Where did you download the games from on both devices?
- Are both versions of your MMR up to date? Do both of the devices have the latest updates for their operating systems?