There is a bug in Netgear R6220 that causes wireless issues, especially on the 2.4 GHz band. On the bug tracker for the issue they have found a fix and to apply it they state:
do a local build with the following change applied and see if if it fixes the issue?
Before flashing an image with this change, make sure you're able to recover the device through TFTP if needed.
diff --git a/target/linux/ramips/patches-4.14/0039-mtd-add-mt7621-nand-support.patch b/target/linux/ramips/patches-4.14/0039-mtd-add-mt7621-nand-support.patch
index d50e689110..03b2b36db9 100644
@@ -3578,7 +3578,7 @@ Signed-off-by: John Crispin email@example.com
-+ shift_on_bbt = 1;
++ shift_on_bbt = 0;
I have searched around on the forum and the documentation, but can't find any instructions on how to do this. I'm sure it's been answered and I'm just searching with the wrong phrases. Can someone point me in the right direction on how to follow the above instructions?
Edit: Are you sure that is what you're seeing? "MTD partition offset not correctly mapped when bad erase blocks present"
doesn't seem to mention wireless issues mentions wireless issues in the comments.
All this report is about problem with reading WiFi EEPROM data from a flash partition "factory".
The openwrt.org site appears down, but the first steps would be getting your own build machine up and running. A Linux-based OS is the easiest, within a VM is functional if you don't have a dedicated box.
Once you get it up and running, you'll want to clone https://git.openwrt.org/openwrt/openwrt.git
If you're not familiar with git, this would be a good time to understand the minimal commands, such as
First, I'd build a "stock" image just to make sure that everything is working as expected.
Once you're ready to go and on your own branch, the patch looks like you'll need to edit
target/linux/ramips/patches-4.14/0039-mtd-add-mt7621-nand-support.patch to make the change noted
Then you can
make -jN clean download world (my normal mantra)
The build system documentation can be found on openwrt.org from the main documentation page, once it's back up. Getting all the prerequisite packages installed on the host is likely the most time-consuming part. That documentation will also explain
make menuconfig to be able to set up the build for your device and give you the option to add more packages.