DHCP client packets from ROS only have a TTL of 16 by default. It would be nice if this value was adjustable, because it is so low by default.

As a comparison:
Windows 10: TTL of 128
TP-Link routers: TTL of 64
Netgear routers: TTL of 64

Mikrotik's DHCP relay will increase the TTL in the process of relaying the packet but not all DHCP relay implementations do this.

Are you seriously saying that your DHCP server is more then 16 hops away from your DHCP client?
Anyways, you can use firewall rules to change the TTL.

you always can mangle it by ROS.
but as above noticed: its had not much Practical purposes.

Hi!

Tried to increase dhcp client TTL using mangle. It just doesn't work. Can anyone help me?

Try mangling the TTL at a hop between the client and server. If memory serves, the DHCP packets originating on a device do not hit the firewall.

There are no intermediate hops, unfortunately.

There are no intermediate hops, unfortunately.

Then why is the TTL interesting at all?