[werk_ag]

...
If you have any more ideas it would be much appreciated. 

Hi Zak,

The upload to ThingSpeak is been working for me since long time, and is still working. Also, I don't have received any complaints about it... however, it is also part of my job following other communities, and more and more, the ThingSpeak Team is recommending people to use their own Arduino library, instead of custom code. Inclusive there are many people reporting that systems that had worked for years are not working so well now. They also confirmed that did changes in some of their servers.
This discussion on the ThingSpeak community isn't new, but the last statement from ThinkSpeak team, again suggesting people to start using their own lib is from 28 Apr 2020.

Besides requiring the API key, seems that some servers now also need the channel number.

Usually I react fast to these news, and I already modified the Pro2 Compact, and AQMII softwares, to work with their library.

On next weekend, I will publish the updated versions.

P.S The calibration of the CO2 sensor seemed to work, but I think it is drifting a bit. Much of the reason for getting ThingSpeak working is to see if the drift is normal diurnal variation that you have discussed in other posts, or if I have a sensor problem.

Remember the error margin of the MHZ19 is +-50ppm, don't expect a completely precise reading. Measuring outside CO2 with a good degree of precision is a complex task, which requires complex and expensive equipment. There are good topics on the forum about this matter.
The main idea behind the CO2 sensor is spreading the sensibility to the importance of being conscientious about the CO2 emissions.