CM1100 and Suddenlink 1G service

[entact]

I have purchsed a CM1100 and have Suddenlink 1G service (Docsys 3.0).

Suddenlink was able to get the modem working but with 2 caveats

1. I'm unable to hook up a Nest Wifi router. It works fine with Suddenlink rental modem. Could this be DHCP issue?
2. It seems to be fine with 50Mb upload but downlood seems to be capped at 30Mb
   
   I know that in 2018 this modem was not compatible but has there been any change (since it is supposed to be backward compatable wiht Docsys 3.0)
3. Are there any settings or directions I can give to Suddenlink to get this working?

[FURRYe38]

The CM1100 is cabable of 1Gb speeds. I know mine is. 

The CM1100 is a DOCSIS 3.1 modem though backwards compatible with 3.0

Be sure you speed test with a wired PC behind the CM modem first with out a router in the mix.

You can connect any external wifi router to the modem. 

Be sure your chosen wifi router is also capable of 1Gb WAN to LAN speeds on a wired ethernet connection. 

Have the ISP check the signal and line quality UP to the modem. 
Be sure there are no coax cable line splitters in the between the modem and ISP service box. 
Be sure your using good quality RG6 coax cable up to the modem. 
Be sure to power OFF the modem for 1 minute them back ON.

[entact]

Yes to all of these

Be sure you speed test with a wired PC behind the CM modem first with out a router in the mix. CORRECT

You can connect any external wifi router to the modem. 

Be sure your chosen wifi router is also capable of 1Gb WAN to LAN speeds on a wired ethernet connection. CORRECT Nest WiFi

Have the ISP check the signal and line quality UP to the modem. EXCELLECT
Be sure there are no coax cable line splitters in the between the modem and ISP service box.  NONE
Be sure your using good quality RG6 coax cable up to the modem. NEW RG6
Be sure to power OFF the modem for 1 minute them back ON.

Getting spotty 250MB to 350MB with rented modem. So suspect: 

1) Defective CM1100

2) CM1100 does not work with Suddenlink
3) Suddenlink not passing correct config to modem

[FURRYe38]

Try a different CM1100 modem. 
Be sure the ISP is sending the correct config file for the modem.