Good to hear the DSL-N66U is still going strong 
As @Grisu details, keep an eye on RAM usage and see if that keeps increasing upto 19 hours etc.
Also try a different browser, just to make sure then if that fails try the below
I would if possible try a factory reset and rebuild router from scratch (not backup) and hopefully that will clear any issues and then see if that helps ?
I have not seen issues with access to DSL-N66U mentioned on this forum, so hopefully just unique to yourself
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