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I have had 2 Cox Techs out here. They found no issues and see 1.1Gbs directly on the line. After the modem I only get ~300Mbs. I contacted Arris and they have stated that their specs for the SB8200 require an Upstream signal power range of 45DBmV to 51 DBmV. I am getting roughly 35DBmV. I spoke to a Tier 2 tech on the Gigablast team who stated they cannot increase said signal power. From my own research online it seems like this isn't true. I posted in my signal readings below, any help would be greatly appreciated.
Current System Time: Thu Jan 10 13:20:45 2019
Only Cox can provide the acceptable range of signal strengths for your particular plan and device...not Arris, not Google.
Generally ranging, upstream power should be between +37 to +55 dBmV but ideally closer to the middle at +46 dBmV. Yours appear to be in the low end but, as I stated, it really depends on your plan and device.
Will the low signal cause my download speed to be 300Mbs or less?
Signal levels look good. Do you happen to have IPv6 enabled? You can tell by doing a tracert or ping to google and look at the IP. If it's the normal 172.217.x.x then you don't. If you do, try disabling IPv6 and see if your speed goes up.