You should be going modem--->router---->devices. 

the S33 is a modem only device. What router do you have? 

Xfiniti cable ->Model 33->LinkSys ModeEL Velop MX4200C (mesh wifi)
 

not-workng Devices take 10-15 minutes of repeated attempts before connecting.I power off the Model s33 multiple times

Same devices work immediately  with Mobile hotspot

Are you sure its not the router having issues? 
Does rebooting the router help? 

Modems tend to either work, or not at all. So if some devices work and not others, its not usually the modem as its just assigning a IP address to the router and the router manages the connected devices

I  understand you are syaing that S33 is ok and all is problem of wifi setup. This problem started when I started to use S33 Surfboard.  Same devices worked without any issues earlier.

Is there a firmware upgrade I need to do ? it ha snot been done on S33
 

How can I login to S33  and see what is going on 

You can access it via 192.168.100.1

Firmware is part of the isp’s provisioning and not users or even arris updateable. And they’re rare to get updates because each update requires certification that costs $. 

What actual notice do you get from your devices that won’t connect? 

OK let me connect via 192.168.100.1

The not-working device does not connect to Wifi (sometimes it even connect but no communication via wifi takes place).It starts to work as soon as I connect to mobile hotspot. 
The LinkSys velop has been connecting for last 3 years without any issue.Only when I replaced it with S33 that the problem started 

If it can’t connect to wifi, its not the modem. Again, it either works or not at all. And it has nothing to do with the wifi. 

pls see above update . what can I do to solve issue. Or if there is a better modem let me know

I don’t believe its the modem. Based off what you described, I’d be looking at the router more than the router. 

Modems are more all or nothing. If a couple devices won’t connect, it the router. If EVERYTHING won’t connect, then it might be the modem.