When I run Cell Monitor, I am seeing the following LTE B28 appear:

445 B28 9360 -104 -18.0 -76 2
220 B28 9360 -105 -19.0 -76 2
122 B28 9360 -107 -20.0 -77 9
216 B28 9360 -107 -20.0 -76 2

While I can connect to all the other bands shown (B3 and B20), I can't seem to connect to these bands for some reason.

I am using But this causes the modem to disconnect from LTE entirely and drop back to a 3G connection. Why can I see them, but not connect?

If I read the lines correctly, then those B28 cells have extremely low signal level (-104dBm and lower), essentially they are useless.

Really? the B3 one I have been on for years is :

87 B3 1392 -123 -15.0 -99 1

RSRP : -123
RSRQ : -15
RSSI : -99

I thought B28 would travel a lot better and be more stable?

RSRP is indeed lower for B3 cell, but RSRQ (an approximation to SINR) is better (-15dB vs. -18dB).

When I used to work (as a senior radio engineer) for a major MNO in my country, we considered RSRP of -105dBm as edge of usable coverage, likewise RSRQ had to be above -10dB or so. When signal strength, reported by UEs (modems, phones), was lower than that, many didn't work reliably any more. Some devices worked surprisingly well at signal strength (and quality) much worse, but some devices struggled even at better signals.

I see very low RSRQ, which seems to cause a handover to the 3G.
But again this depends on the settings of the cellular network. Some operators use the RSRQ as trigger for intersystem handover, other not.
Your B28 has low RSRQs relative to the RSRPs, which means that it receives external interference. Raise a complain to your service provider.

Your B28 has low RSRQs relative to the RSRPs, which means that it receives external interference. Raise a complain to your service provider.

I don't even know where these signals are coming from! I have a Pixel 6a running CellMapper and can't seem to get it to record these cells. I've been to the locations of the masts I know about and can't see them there either!

Could they be 'roaming' signals from another network provider?

Try find them on https://www.cellmapper.net and you will see to which provider belongs.

If channels are shown during normal operation (LTE modem is registered to network, traffic flows), then they are somehow candidates to be used (either mobility or CA or both). If they only appear while modem is searching for network to connect, then they may be just anything.

I'm still wondering why you're so much into that band (and cells)? They have low signal strength and shitty quality so even if they belonged to your MNO, service wouldn't be something to hope for.

Try find them on https://www.cellmapper.net and you will see to which provider belongs.

I have, but it's a rural area and I'm mainly the only contributor of data to CellMapper, to date I have not connected to these bands, so they are not on CellMapper

If channels are shown during normal operation (LTE modem is registered to network, traffic flows), then they are somehow candidates to be used (either mobility or CA or both). If they only appear while modem is searching for network to connect, then they may be just anything.

I'm still wondering why you're so much into that band (and cells)? They have low signal strength and shitty quality so even if they belonged to your MNO, service wouldn't be something to hope for.

Because my provider is Three UK and in a rural area like mine these bands are a life line and are the difference of getting internet or not. Their Band 20 coverage is really good but they only have 5Mhz slice of it, compared to 15Mhz @ B3 and 10Mhz @ B28

The entire reason for going with Mikrotik products is that I can band lock and have more control over what mast and bands I am connecting to. Just can't figure out where these B28 signals are coming from, as according to Cell Mapper the nearest one is 30 miles away and behind a mountain :/