I do have collectd-mod-thermal installed but I don't see any temperature graphs. 
I'm not really familiar with collectd, but you could check /sys/class/thermal just to see if there's anything in there (should be thermal_zone0 or something similar I think).
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OpenWrt 21.02.3, r16554-1d4dea6d4f
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root@OpenWrt:~# cd sys
-ash: cd: can't cd to sys: No such file or directory
root@OpenWrt:~# /sys/class/thermal
-ash: /sys/class/thermal: Permission denied
root@OpenWrt:~#
Try ls /sys/class/thermal
root@OpenWrt:~# ls /sys/class/thermal
Hmm, if there's nothing there I assume (just a guess) your router doesn't have that capability, I had a similar problem with my linksys wrt1200 and the cpu frequency plugin, installed/enabled, but it just never showed up, the only thing in logs was that collectd couldn't find a cpu.
lm-sensors maybe?
As far as I know, none of the thermal-sensor kmods are included by default. You'll have to install the ones that are applicable for your SoC yourself.
How to do that?
Edit:
Are you sure that SoC even has a readable, built-in temperature-sensor in the first place? Most of the SoCs I've encountered do not have anything like that, since it's typically not a feature that's needed on consumer-grade routers/access-points.
I'm too tired to start digging into it myself and I've got a long drive ahead tomorrow.
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No problem. I am in no hurry to sort this out. Just need the right directions.
Search if your device
(I doubt it).
A forum search might yield some useful information.
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It doesn't.
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Is it a MIPS processor?