The patch just adds two lines to the pppoe kernel module, so you can just build the module and insert it into the running OpenWRT system:
- stop ppp (it uses the kernel module when connected): ifdown wan
- rmmod pppoe
- insmod /root/new_module/pppoe.ko
- ifup wan
Just to be sure, you can use the verbose patch (" patch with debug output "), it prints a message when it is loaded (or first used, not sure), just so you can see that the patched module is in use (sometimes insmod loads the original module, from the /lib/modules/ folder, not sure why...).
Also note that you must use the stripped module file, not the one in the build folder! That one might crash your router.
Just compare md5sum of unchanged modules with the installed ones, to find the folder with the "correct" version.
(sorry for this if you are an openwrt build expert, it confused me on my first tries)
PS: the debug version also writes a log when a PADT is recevied, noting the destination address, so you can see of "wrong" packets arrive on your connection. (if they don't then the reason for your problem might be something else)