Hello MikroTik Community!

Simplified diagram of my network is attached below.

After restarting the devices (e.g. as a result of a power outage), computer 1, which is plugged directly into the router gets immediately access to the LAN resources and WAN (Internet). However, for some reason computer 2 gets access to the local network immediately, but access to the internet is possible after a few minutes!

What could be the reason for this? Could it be an RSTP/STP configuration?

With what you describe it's not STP/RSTP (minutes, LAN OK but no Internet)
But it can be 100 other things. Start by looking at the LOG.

During the time when internet access is unavailable, I would:

• ping the gateway/router,
• ping the other computer (should be on the same LAN, right?)
• ping some public IP (e.g. 1.1.1.1 or 8.8.8.8 )
• try to resolve some domain with default DNS (e.g. nslookup google.com)
• try to resolve some domain with specific DNS (e.g. nslookup google.com 8.8.8.8 )

Then try to do the same once internet access is available. There must be a difference and the difference will point out where is the issue