In my home, i am using Coaxial Cable Internet System with Docsis 3 0 protocol and it uses mac id verification to get Internet and real iip from the main servers of my ISP... I wanna set my cable modem to bridge mode and i will set my Mi 4A Gigabit Edition device as a cable and wireless router. I will make some filters on Internet with this router device and also i gave ip local addresses from this router with DHCP that working on Mi 4A Gigabit Edition. Also i will install vpn on this router and i also use this filtered clean Internet with my gsm phone in the outside of my home then...
In this scenario, how can i set my Mi Router 4A that installed OpenWRT as a wired and wireless access point?
It sounds like you'll be using this as a normal router, assuming that you do indeed put your ISP device in bridge mode. So basically, it should 'just work' in the default state since your ISP is likely providing an address via DHCP.
When you install the new router, you'll need to reboot your cable modem, possibly a few times, such that it gets an IP from the ISP.
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Dear @psherman;
First of all, thanks for your quick reply. In addition, also i need to copy my Original ISP modem's wan mac id.
How can i do that?
Just by adding a string to my "/etc/network/config" file in openwrt ssh console?
Thanks again... 
It would likely be your old router's wan MAC. Once you know what it is, you can simply add it to the device definition of your wan port.
However, this isn't entirely necessary in most situations -- the modem reboot usually is all that is required for the vast majority of the DOCSIS services out there (unless your ISP is different).