This doesn’t make sense at all. You should fix the problem, not mess with the module’s internals.
Combining information from your above posts, it seems mac join abp works:
And that at some point (or right away?) mac tx fails:
Right?
So, what happened between the successful mac join abp and the failing mac tx?
The RN2483 cannot know if the session configuration you set before running mac join abp is correct. So, it can only return accepted for mac join abp, regardless the values that you set for DevAddr, NwkSKey and AppSKey. Even more: it won’t transmit anything at that point; it’s just a means to tell the device that you claim that you configured the device properly. (Remember that “ABP” is “Activation by Personalisation”, so: activation by just configuring a fixed “session”. Indeed, not a “real” join like @cslorabox says, and quite a bad choice in naming of the RN2483’s commands.)
Next, when transmitting at some point later, it still won’t know if the DevAddr and session keys that you set are the ones known by TTN. It will just assume you set the correct values, use those to create a LoRaWAN packet, transmit that encrypted packet, hope that one or more gateways receive it, and that the backend servers can handle it. There’s no reason to throw any error at that point.
So: I’d assume you either did not set all of the ABP parameters, using mac set devaddr, mac set nwkskey and mac set appskey before calling mac join abp. (If true, then it’s weird that it does return accepted at that point but still throws not_joined later on. But who knows.) Or that you power cycled or somehow reset the RN2483 between running mac join abp and mac tx?
And asides:
Why are you not using OTAA rather than ABP? In EU868 OTAA will also get you the correct network settings in the OTAA Join Accept, which you need to set manually for ABP. Also, the frame counters are reset automatically for each join, which is helpful during development. (See How often should a node do an OTAA Join, and is OTAA better than ABP? And see OTAA shows "Activation DevNonce not valid: already used" for a workaround for OTAA issues during development.)
And note: