Hi,
I have mikrotik router (RB750Gr3). I would like to set fixed IP addresses of my wireless devices. But router has not wifi. I use unifi AC-LR, but there is not possible to fix IPs.
Recently my device changed its IP by itself, which is not good for our smart home.
Which device is acting as DHCP server on your network ?
There you should be able to set fixed IP.
/export file=anynamyouwish ( minus router serial number, any public WANIP information )
Now you’re exaggerating 
According to all mikrotik. Unifi is only an AP, fixed IP addresses cannot be set there (I was told on the unifi forum).
There is no other router in the network.
I’m sorry, where should I export it from?
On your central router you can see which device got which DHCP lease.
In the lease tab you can make it static, changing the address which should be used for that specific device.
That what I call the Belgian waffle jajajajajajajajajaja
What follows is the Canadian pancake with extra Maple Syrup 
…
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Find the lease you want to make static and RIGHT CLICK on it to get the pop-up menu!
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And what about “no spoon feeding” ?
Well somethings need a little gagging, dont you think 
Specifically, and get your mind out of the gutter, I am referring to things like: “where is that damn set static lease setting anyway”, or the infamous, “where the heck is that damned static DNS setting”, thats on every config that no one seems to figure out how to remove.
Fork feeding … at least I’m usually using fork (and knife) when fighting with maple syrup
Something like this?
/ip dhcp-server lease
add address=192.168.88.2 mac-address=08:09:0a:1e:52:CD server=server1
add address=192.168.88.3 mac-address=08:09:0a:1e:52:C0 server=server1
You use terminal in Winbox application.
UniFi Access Points have a default IP of 192.168.1.20 until they are “adopted” by your controller application or cloud controller.
I am not an expert on UniFi as I have moved over to all MikroTik.