Seems like source of the problem is found, just need to confirm that and push it to the staff.
After sniffing for DHCP packets and comparing them I’ve noticed that IPv4 packet Destination for DHCP Offer packets when success is always offered IP and when fail is a 255.255.255.255 AND Ethernet II destination is MAC of requesting device when success and Broadcast ff:ff:ff:ff:ff:ff MAC then fail. That’s very logical because client device at this moment is listening to packets for it’s own MAC and not for everyone.
So DHCP server sends offer unicast messages to clients by always known MAC’s and it’s works perfectly but when server sends offer as broadcast message then client ignores that.
PSS: finally I’ve got a chance to collect some information from a device (camera) and found that after connection to MT RB2011 there are lot of “RX error: Dribble bit”. Then added to it that always device connects at 10/Full (with a lot of linkdowns etc). When I got to the camera internals I’ve access to kmsg… that’s how I’ve seen that errors. I got ifconfig info and understand that camera easily can connect at 100/Full. Then after connection to other brand routers I’ve rechecked all this and found that camera connects to all of them at 100/Full… And understood that DHCP offering lease error is not the main problem, the main problem is negotiation and searched a bit for that. And found a topic from 2013 with same problems with lot of devices - Cisco, Huawei, ECI and others… There are posts from 2021 too.
Before this I was thinking about buying new RB4011iGS but… after all this happens and I’ve beating the wall with my head for more than a month - I start to think that I must finish think of buying RB4011… 