@Gytis I was having the same issue with my new Mikrotik Lora9 kit. The instructions from Mikrotik say it is important to enable legacy forwarder which as pointed out already is not part of TTN V3.
htdvisser had pointed out the following guide:
https://www.thethingsindustries.com/docs/gateways/mikrotikrouterboard/
After reviewing it I noticed that in the Mikrotik OS Lora config page, when configuring the device, there is a dropdown you need to click and select “Network Servers”. Once I selected “TTN V3(nam1)” (I’m in latin america), the gateway showed up in my nam1 group console.