Master 1.3.1 - Data Rate Fix

[tohlh]

Greetings, I have been testing this firmware since last month. I have noticed some bugs related to data rate (DR) and decided to investigate.

This is my comment on PR #78 of arduino-libraries/MKRWAN:

Every time, when I try to send the payload on the first attempt, I am always limited to 11 bytes, regardless of what data rate I am on.

On the second attempt, the limit will always be maximum payload size - 11 bytes.

On the third attempt, the limit will always be maximum payload size - 8 bytes (which is normal).

It turns out that there are a few factors causing the above to happen:

1. When we change the DR with AT+DR, the DR will only change at the second AT+SEND command. It does not take effect on the first AT+SEND immediately after setting AT+DR.
2. When we issue AT+JOIN command, the DR is automatically restored to the default data rate. In my case, it is DR2. This is why I was always stuck at a maximum payload size of 11 bytes immediately after I join the network.
3. For the second attempt, my device was responding with a 3 bytes MAC command issued by my server, hence this is not a problem to fix.

The codes I added will change 2 things:

1. Make sure the data rate is changed immediately for the next AT+SEND whenever AT+DR is issued.
2. After successfully joining the network, restore the DR to the one previously configured by the user, not to the default DR.

What I am trying to achieve with my modifications is to ensure a responsive and consistent DR in control of the user. Hope my modifications above helps. Thank you.

Co-authored with: @kygoh

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