I bought a brand new AP...
I Can not conetct (first time) to set up the router. The winbox can not see as 'Naighbor'.... I plug it directly into the computer.
While I connect it into the working infrastructure get IP addres from dhcp... (on ETH port).. But can not connect any way (winbox, webrowser, ssh).
I tried to connect directly via mac address in winbox, nothing too.

While I write this lines... I use wifi, I can see the other devices in the network... but the new not...
Its mac: 48:8F:5A:5B:C1:1E

Thanks before

use / plug it into your infrastructure with another port than ETH1/WAN...standard setup is as such, that ETH1 is WAN with DHCP-Client enabled....but there is a firewall behind it.
All other, starting from ETH2 should be fine and Winbox should detect it.

Depending on what wAP ac you have there is or there is no ether2. If there is ether2, then OK this is a LAN port and should be OK for configuration access.
If there is no ether2 (MIPSBE older version, RBwAPG-5HacT2HnD, not RBwAPG-5HacD2HnD) , then you have to enter via wifi, and "add ether1 to the bridge". (Ether1 will become a LAN port, not a WAN port blocked by the firewall). Or any other action that will remove the management denial of an interface in the "WAN interface list". (Remove firewall rules, add extra allow rule, change interface list membership , .... etc etc)