I have Netgear CM1150v which has never given upstream speeds over 18 mbps for contract 20mbps

The CM1000 and 1100 series only supports up to 1Gb due to the WAN Ports are maxed out at 1000Mpbs. Nothing beyond that. Most ISP support a 1Gb speed tier. However CC and XF last year started a mid split in there new speed tiers that they set up for 800Mpbs and a 1.2Gb speed on the down side. No idea why they couldn't keep the 1Gb tier however probably best way for them to support the higher 1.2Gb speeds they started pushing. Yes this also caused problems on the user or user owned modem side of things and the 1Gb supporing modems could not get the full 1Gb speeds anymore and had to fall back to 800Mpbs for those 1Gb supporting modems as 1.2Gb was not achievable by these modems. This is only seen with CC/XF ISP. My ISP Sparklight still has a 1Gb speed tier service. No idea if or when they will push that beyond.

So you have an option to ask the ISP for the lower speed 800 tier which this modem would be supporting of, or get in to a newer CM2000 series or wait for the awaited CM3000 series modems that have WAN ports that start at 2.5Gb connection rated speeds and then you could get the 1.2Gb or higher speeds that seem to be coming out by the ISP. No idea if or when the CM3000 will be officially released to market. NG doesn't publish that info. So you'll need to wait for find another brand that suites your needs.