I'm using Mikrotik router in the normal "Router" mode (I think it is called Home AP in quickset)
NAT and DHCP enabled and everything works fine when I connect to Mikrotik via wifi.
I also use my Linksys router (AC1200) to extend the coverage and I set it up with "Wireless bridge" mode as explained here https://www.linksys.com/us/support-arti ... Num=141083
The problem is then I connect to Linksys via ethernet or wifi the clients are not getting an IP address from DHCP and I have to configure static IP (then it works)
Any idea why? This setup worked just fine with Dlink router I used before I replaced it with Mikrotik.

Post the configuration export from the Mikrotik (see the anonymisation hint in my automatic signature right below) and indicate to which port the Linksys is connected.

Mikrotik is basically in the default configuration with quickset option "Home-AP Dual"
Linksys is connected via wi-fi to wlan2 on Mikrotik (5Ghz) not sure what do you mean by port.

I never use quickset so I have no clue what "basically the default home-ap dual" means exactly. That's why I ask for the export of the actual configuration.

It wasn't clear from your OP that the Linksys itself is connected wirelessly, so does it act as a wireless repeater (some manufacturers call "wireless bridge" a device which bridges Ethernet interfaces with the wireless one, so I thought it was that case)? If so, the issue may be that all devices behind the Linksys are seen by the Mikrotik as having the same MAC address (due to the way how the standard wireless protocol looks like), so the limit of max. one DHCP lease per MAC address may prevent them from getting IP addresses since the Linksys itself got one. So check this setting on the DHCP server on the 'Tik.

Point is : what is the implementation of a wireless bridge in this Linksys?

After reading the mentioned doc it seems to be like the Mikrotik "pseudo bridge" , also known by others (like ENGenius, Draytek,, TP-Link ....) as "Universal repeater".

There are multiple implementations possible for an universal repeater. It's about handling of the MAC address. The repeater in the wifi channel either has to replace the source MAC address by its own MAC address, to get the answer back , and then forward that answer with the original MAC address. , or it has to do proxy-ARP and have the answer coming its direction.

It looks like the Linksys is doing "pseudo bridge", also known technically as a 2.5 layer bridge. A bridge/switch is OSI layer 2, and does not touch the MAC address. This one does. Wifi needs WDS to have 4 MAC addresses in a packet, but vendors have different WDS implementations.

This works in many combinations, however not with the Mikrotik DHCP server ( see viewtopic.php?f=13&t=133784). For one or other reason the DHCP verification does not reach the client device behind the repeater, and the DHCP lease is dropped. Using another brand DHCP server makes it work.

At least that is my experience.

Yes my understanding is the same.
And it works fine if I do not use DHCP on the client devices.
So I was wondering if there is a workaround on the mikrotik to make that DHCP packets get through the wireless bridge.
I tried to play with setting in the "DHCP server" and it sometimes worked and client received the address but I was not able to make it work reliably.

Did anyone report this to Mikrotik support?
I'm trying to do this but not able to register on their support portal

Just send an e-mail to support@mikrotik.com. There should be no need to register at the portal in advance.