LoRa Class C device can't retry-join in OTAA mode

[cyliangtw]

Description of defect

Normally, LoRa Class A device could retry join several times in OTAA mode.
As while I set LoRa device as Class C before OTAA handshake lora.connect(), if RX1 can't receive 1st join-accept packet, it will stay in the state "RX2 slot open" and not proceed any retry-join procedure.

Target(s) affected by this defect ?

Not depend on the specific target.
I could reproduce this symptom on both of NUMAKER_IOT_M487 and NUMAKER_IOT_M252.

Toolchain(s) (name and version) displaying this defect ?

GCC_ARM

What version of Mbed-os are you using (tag or sha) ?

OS v6.15

What version(s) of tools are you using. List all that apply (E.g. mbed-cli)

mbed-tools 7.2.0

How is this defect reproduced ?

Modify lora example https://github.com/ARMmbed/mbed-os-example-lorawan, to add set_device_class(CLASS_C) in main() of main.cpp as:

    printf("\r\n Adaptive data  rate (ADR) - Enabled \r\n");
    lorawan.set_device_class(CLASS_C);  //## Mbed support class A and class C; OTAA should be in class A.
    retcode = lorawan.connect();

[mbedmain]

@cyliangtw thank you for raising this issue.Please take a look at the following comments:

It would help if you could also specify the versions of any tools you are using?
How can we reproduce your issue?

NOTE: If there are fields which are not applicable then please just add 'n/a' or 'None'. This indicates to us that at least all the fields have been considered.
Please update the issue header with the missing information.

[cyliangtw]

Update the missing info:
What version(s) of tools are you using. List all that apply (E.g. mbed-cli)
mbed-tools 7.2.0

How is this defect reproduced ?
Modify lora example https://github.com/ARMmbed/mbed-os-example-lorawan, to add set_device_class(CLASS_C) in main() of main.cpp as:

printf("\r\n Adaptive data  rate (ADR) - Enabled \r\n");
lorawan.set_device_class(CLASS_C);  //## Mbed support class A and class C; OTAA should be in class A.
retcode = lorawan.connect();

[Shiny380]

@cyliangtw I got around this by joining in Class A mode and then switching to Class C after a successful join. I poked around in the lorawan source for a while to try and fix this but required too many changes in too many places. If I get time soon I might continue with the fix and open a PR but not sure if it's worth it.