Management interface not always accessible (trouble with settings)

Appears there are very basic misunderstandings of what OOB is and how it can be deployed. Lot to learn sorry saying.

An OOB network is a dedicated, standalone independent network to keep the management access running, even if all other links and connections are (for example) of the production network are not operational.

And the first thing you do is interconnect the OOB and the production network, and try to bring it into the same IP subnet?

Large scale networks have a dedicated network infrastructure: Complete physically and logically independent networks, to retain the management access. Historically, this network was used to keep the access to the physical typically serial consoles. Over time, sophisticated services were added to this layer, like IPMI for server monitoring and management. IPMI is a series of specifications that provide standardized interfaces to the so-called “platform management” services. In this context, the term “Platform Management” refers to monitoring hardware (system temperatures, fans, power supplies and so forth), their control (booting and shutting down the server) and the documentation (logging) of “out-of-range” states. 

Doing so, requires to have dedicated subnets and IP addresses for this network.

This is is neither an error nor a behavior 8-) - it's design, and all intentional.

Start your studies here: https://en.wikipedia.org/wiki/Out-of-band_management