If the problem occurs when connecting via mobile data, it indicates that there are multiple cell towers in your area. When you try to enter the game, your connection switches from one tower to another as your character loads, causing the connection to drop and preventing you from entering the game.
To avoid this issue, please try to establish a stable connection. For example, if you can connect to a Wi-Fi network, try logging in through that or install a VPN on your device to keep your connection stable.