Wifi not connecting after Altice box update

It's not possible to "combine" 2.4ghz and 5ghz without some pretty advanced networking hardware on both ends which no phone, tablet, computer, or home based router can normally do.  They likely meant that they are now using the same SSID for both.  If this is the case it's the cause of your problem and needs to change.  If the manufacturer has decided that separate SSID's is no longer an option, find a better router because that would be a very poor decision on their part.

Your simplisafe system will only work on 2.4ghz, so agree, a new router may be warranted.