loss of connectivity, slow RSSI level

for past 2 months, our laptops have been experiencing connection drops and poor RSSI levels after 5 pm (over 80 dbm) - Mornings seem good (40-60 Dbm).   Before March, never had an issue. We do not stream, do not play video games.  just basic internet use, some work but mostly email.  Many momentary connectivity drops, and overall slow downs unless we are within 8 feet of the router/modem.  I have an ARRIS surfboard which worked great for past 18 months...its still in stores so its not an outdated model.  I can monitor RSSI levels on the ARRIS admin site...definitely different levels over past 2 months.  Xfinity's solution is to reset the modem- I have done that a few times with no significant change.    I have adjusted ARRIS settings to maximize RSSI and overall performance but have reached my limit of understanding.   

I am considering extender to improve performance but not sure that is the whole issue.  Don't want to spend the money for no help.  Tried the chat feature with xfinity but kept going in circles and recommending reset...good luck talking to a person!