As a rough rule of thumb, mips based devices (SOCs, such as your mt7621) are relatively unlikely to sport thermal sensors for the SOC. The mips architecture is old and hasn't really been updated (arch-wise) in around 20 years[0] (yes, Imagination Technologies tried to get a foothold into early Android, but that failed and never ended up in router SOCs), there generally is no adaptive frequency scaling, no sophisticated PMU (ower management unit), no need for thermal control (max power consumption and heat are 'predictable' and less dynamic).
Modern ARM on the other hand usually has thermal sensors, but that doesn't necessarily mean that OpenWrt will support or expose them (but that is likely 'fixable').
'All' wifi6 or newer wireless cards are likely to have some sort of thermal monitoring capability (wifi5 might have it), they really need it to protect themselves - likely the radios are running hotter than the SOC itself.
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[0] you may actually extend this to the mid- to late 90s, when SGI/ mips effectively died.
I am sorry for the long delay. I have been busy.
Thank you for the detailed explanation.
As I mentioned in my reply to brada4, turned out my router has sensors for the radios.
The collectd-mod-sensor plugin works for them.
I have another router, a Linksys EA6200 with DD-WRT. Its CPU temperature is near 100 ℃ without cooling!!
With a laptop cooling pad it is 70 ℃ on average.
For the AX23, I have been monitoring the radios temperature for a while and it is 59 ℃ on average for both bands and, as you explained, it is likely that the SOC is cooler.
Looks like the AX23 doesn't need external cooling.
I will keep monitoring.