Downstream issues

I recently switched plans (from 175Mbps to 600Mbps) and then a couple of weeks later switched modems from Motorola SB6121 (4x4) Motorola MB7621 (24x8). Since the modem switch, I've had all sorts of trouble, most pragmatically extremely fluctuating download speeds (between 1 and 90Mbps, which is slower than the 135Mbps that I used to get with the older modem), along with short disconnections throughout the day.
Looking into this more, the downstream channels aren't bonding, many of the channels are not locking, power levels are out of spec and some channels' SNR are too low. I also have hundreds of thousands/millions of uncorrected errors. There's also a wide range on the power and SNR levels.
Upstream looks fine, if a little on the higher end on the power levels. The upstream connectivity is quite steady and locked right around 17Mbps regularly, no matter what the downstream is like.

 

I've read through the long and very helpful Internet Troubleshooting Tips thread along with https://pickmymodem.com/how-to-fix-your-docsis-3-0-3-1-signal-issues and https://pickmymodem.com/signal-levels-docsis-3-03-1-cable-modem and https://www.motorolacable.com/documents/CableConnection.pdf

 

I also connected the modem directly to the cable outside my house (to the grounded connector) and that did not help much. I basically have a home run from there to the modem's usual place inside, with no splitters at all, just a couple of 1:1 connectors and the wallplate connector.

I've included numbers below of what it looked like when connected directly outside. Though the power levels are above -15dBmV, when inside they drop to between -15 and -17. This is after about 7 minutes of uptime, note the large number of uncorrected errors, some of which are more than the corrected ones. Usually channels 19-21 are not locked, but sometimes others are not locked too.

(You may need to scroll right to see all the columns)

 

Startup Step Status Comment           Acquire Downstream Channel 543000000 Hz Locked           Upstream Connection OK Operational           Boot State OK Operational           Configuration File OK d11_m_mb7621_extremeproplus_c01.cm           Security Enabled BPI+

 

Channel	Lock Status	Modulation	Channel ID	Freq. (MHz)	Pwr (dBmV)	SNR (dB)	Corrected	Uncorrected	Uncorrected/Corrected
1	Locked	QAM256	21	615	-10.9	40.8	0	0
2	Locked	QAM256	2	489	-13.1	0	4703	23237	494%
3	Locked	QAM256	3	495	-13.1	38.9	0	0
4	Locked	QAM256	4	507	-13.3	34.7	0	0
5	Locked	QAM256	5	513	-13.3	38.4	0	0
6	Locked	QAM256	6	519	-13.6	29.9	1654688	107419	6%
7	Locked	QAM256	7	525	-13.4	0	0	0
8	Locked	QAM256	8	531	-13.1	0	0	0
9	Locked	QAM256	9	543	-12.7	31.8	357319	21259	6%
10	Locked	QAM256	10	549	-12.9	39.2	0	0
11	Locked	QAM256	11	555	-12.4	30.8	29	0	0%
12	Locked	QAM256	12	561	-12.2	38.5	0	0
13	Locked	QAM256	13	567	-12.1	0	79116	179177	226%
14	Locked	QAM256	14	573	-11.6	0	156451	720202	460%
15	Locked	QAM256	15	579	-12	33.3	14933	104	1%
16	Locked	QAM256	16	585	-11.5	36.4	0	0
17	Locked	QAM256	17	591	-11.6	36.7	0	0
18	Locked	QAM256	18	597	-11.2	38.5	0	0
19	Not Locked	Unknown	0	603	0	0	0	0
20	Not Locked	Unknown	0	609	0	0	2240159	478860	21%
21	Not Locked	Unknown	0	483	0	0	0	0
22	Locked	QAM256	22	621	-11.3	40.3	0	0
23	Locked	QAM256	23	627	-10.6	40.9	0	0
24	Locked	QAM256	24	633	-10.7	41.9	0	0
				Min	-13.6	29.9
				Max	-10.7	41.9
				Range	2.9	12
				Average	-12.2	36.9

 

Channel	Lock Status	Channel Type	Channel ID	Symb. Rate (Ksym/sec)	Freq. (MHz)	Pwr (dBmV)
1	Locked	ATDMA	2	5120	32.4	47
2	Locked	ATDMA	1	5120	38.8	48.1
3	Locked	ATDMA	3	5120	26	47.2
4	Locked	ATDMA	4	5120	19.6	45.8
5	Not Locked	Unknown	0	0	0	0
6	Not Locked	Unknown	0	0	0	0
7	Not Locked	Unknown	0	0	0	0
8	Not Locked	Unknown	0	0	0	0

 

I should mention that on the Comcast Status Centre, it says that it can't connect to my modem, although it's able to successfully do a remote restart, though this doesn't help.

We're having trouble finding a signal

 

I've power cycled the modem numerous times and have talked to Comcast support on chat several times, who were not able to make any progress.

I'm thinking that at this point, I need to schedule a technician to come and take a look at the external line - is that right, and if so, what's the best way to accomplish that?

Thank you!