Hi,
First, what Pau said.
I would add that I think it's unlikely you're actually seeing a direct correlation between your ignored RACH and the notification that the PCU is not connected. Ah, If only I had a satoshi or go on, even a penny, for everytime I've seen an assumption that a "normal" log message is related to a problem being observed.
Normally I would expect to see that log due to a RACH for packet access, (more logging at level debug would tell you) and I was going to ask if you have GPRS configured in your osmo-bsc config to something other than none, but then thinking about it, IIRC, we modify the System Info when the PCU is disconnected and don't advertise GPRS on the cell anyway, so you shouldn't be getting packet access requests. Doesn't matter anyway, It's not the cause of your lack of success with USSD
Ah!!.. second thing just jumped into my memory from osmo-trx days, if you turn up rx-gain enough or have some RF noise generation around, you might be getting a whole lot of fake "RACH" in the logs.
I don't have and have never used a B205-mini, and I haven't even read the specs, but the B200 was just mentioned on IRC and so I thought it might be worth telling you that years ago, I used the USRP N210 with osmo-trx-uhd/osmo-bts-trx and my observation was:
GPS Locked clock = reliable operations.
No GPS locked clock = all kind of problems, not least of which being, precisely what you describe, BTS not "seeing" the RACH. (or maybe no UE response to the assign)
I wonder, come to think of it now, maybe you have set osmotrx maxdly? I'm not sure what the default is, but maybe, you might have more luck increasing it. but that's just something that occurs to me right now. Probably not going to help if you're that far off.
In the end, you need to do what Pau said. In fact, if you log the relevant levels at debug and capture and inspect some traffic, you'll probably have a much more clear idea of what is happening.
Also, then you can inspect your SI, either with gsmtap or off-the air with osmocom-bb or an rtl-sdr/grgsm_livemon, you will confirm if you are advertising PS or not.
k/