I have had internet through Telmex for quite some time. I have one main internet modem from Telmex and have connected an additional wireless router to improve access throughout the house. ...
It could be heat, or it could be due to the new telmex modem not playing well with your router, or upstream telmex changes that were introduced as a part of the fiber upgrade. Here's some diagnostic-type stuff to try if you want:
If overheating is the problem, you can probably fix it by taking the cover off and blowing all the dust off the insides of the thing with a can of compressed air. If you can easily get the cover off, that's the easiest thing to try first. If getting the cover off requires special screwdrivers or voids the warranty or something, that's different.
When the problem happens, see if you can connect to your router using a browser following the instructions to get into the router's configuration utility. Something like put "router.login.net" or 192.168.1.1 or something into the browser location bar. Specifics vary by router manufacturer, so you have to read the manual. (if you don't have a hardcopy manual, prepare by reading an online one while you can).
If your router has crashed due to overheat, I'd expect you not to be able to talk to it at all. If you can log into it, you can check the detailed status screens and get an idea of whether your router thinks it's connected or not. Its possible that the internet connection per se is fine, but that the DNS server that you are being connected to that translates domain names to IP addresses has stopped responding.
For example, it may be that just as a side-effect of the upgrade, you're now using a different DNS server than you were before, and the one you now have is flaky and is causing your problems. You can diagnose this by using a command prompt and 'ping' and 'tracert' (on windows) to see if you can get to internet locations by IP address.
I used 'ping cnn.com' just now and it translated that to 151.101.1.67. So when things are acting up, you can try to "ping 151.101.1.67" and "ping cnn.com" and see if the first works and the second doesn't - that would implicate your DNS server being the problem.
It could be something like your router not playing well with the DHCP service on telmex's new router/modem. If you can get into your router and everything looks to be working, but it can't talk to the telmex router, then look around for a 'release/renew' button and if you can find that and it fixes the problem temporarily then that's the issue - the problem would be that the IP address that the Telmex router has granted to your modem is expiring, and not getting renewed, and after that you can't get to the internet anymore.