Hello,

I have Mikrotik panel Antenna 2.4 Ghz, with
RouterBoard RB 112 and PCI Atheros R52 Card, which supports dual band (2.4 ghz and 5ghz).

can I use this device to connect to Ubiquiti 5 Ghz?
When I scan from winbox, I see all available 5 Ghz signals... but does the other side device accept my mikrotik as it uses 2.4 ghz antenna?

Thank You

well, if you are close enough it might work but I would not do that, just go and buy a proper 5GHz antenna and also replace the pigtail with 5GHz certified cable.
Of course also depends if Ubiquiti AP has AirMax enabled or not, if yes then you cannot connect even with 5GHz antenna.