So here's a method for fixing your MAC address issues properly via persist (that is, as long as something doesn't break your persist/wlan_mac.bin again).
Please note! This guide is for s2, but you can use this guide for other devices, but instead of wlan_mac.bin being in hex it could be normal text or some vendors might use something completely different.
You can optionally use a hex editing app on your PC (like GHex on Linux) or an app like Hex Editor Free on the Play Store.
Download this blank wlan_mac.bin that we will be editing
Now, we will select a MAC address prefix for Lemobile (the company name that LeEco used for s2). According to a MAC Address/OUI Lookup we have three choices:
Using this MAC Generator website (or you can use one of your choice) and enter one of the three MAC address prefixes of your choice from the top.
We will need to create FOUR MAC addresses as that is what s2 stores in its wlan_mac.bin. It is recommended to keep them similar with only the last digit changed as that's what s2 wants.
For example, I'll use C825E1 as my MAC address prefix in that MAC generator. I generated... C825E1BCA0E9, but I want to start my MAC address with a 1 at the end, so I'll do this for my four MAC addresses.
Using the hex editor of your choice, Hex edit the downloaded wlan_mac.bin and from beginning to end? Enter your four new MAC addresses into the HEX fields and not the strings. Please note how I already put in a 1, 2, 3 and 4 to mark the last digit of each of the four MAC addresses. We must OVERWRITE the hex values in there, so please make sure you don't accidentally enable insert mode on whichever hex editing app you use.
Save it and make sure the new wlan_mac.bin is copied somewhere onto your s2.
Reboot to TWRP and mount persist.
Go into TWRP's file manager found under Advanced > File Manager.
Browse to /persist and select your defective wlan_mac.bin and either delete it or copy it somewhere to your user data if you want to keep a backup of it.
Copy the good wlan_mac.bin to /persist
(optional) Go to the new /persist/wlan_mac.bin and select it and chmod it to 0644
Reboot to System and check your MAC address in System > About Phone and if your changes were correct then your new MAC address will appear there.