I finally got around to installing 1.5.9. I'm running it an RPi 4 with an external SSD. I flashed the SSD with 1.5.9, and went through my usual post-update steps using the app. I have a TrendTech/BrosTrend AC3L wifi adapter which requires using the terminal to install drivers. I normally don't have any problems. This time I did. See below for a full copy/paste of the terminal text. Tried a sudo apt update and essentially got the same error public key error. I flashed SM 1.5.6 to a card and tried again. No problems there. So something changed between 1.5.6 and 1.5.9 updates (I did not update 1.5.7/8 by flashing the SSD) that is now giving me that public key/headers package error. Copying and pasting terminal code is the extent of my RPi skills, so this one is way beyond me. Any help is appreciated! Thanks.
stellarmate@stellarmate:~ $ sh -c 'busybox wget deb.trendtechcn.com/install -O /tmp/install && sh /tmp/install'
Connecting to deb.trendtechcn.com (126.96.36.199:80)
Connecting to deb.trendtechcn.com (188.8.131.52:443)
install 100% |********************************| 16512 0:00:00 ETA
Root access is required
Package manager is: apt-get
Get:1 mirrors.ocf.berkeley.edu/debian buster InRelease [122 kB]
Hit:2 raspbian.raspberrypi.org/raspbian buster InRelease
Hit:3 ppa.stellarmate.com/repos/apt/stable buster InRelease
Err:1 mirrors.ocf.berkeley.edu/debian buster InRelease
The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 04EE7237B7D453EC NO_PUBKEY 648ACFD622F3D138 NO_PUBKEY DCC9EFBF77E11517
Reading package lists... Done
W: GPG error: mirrors.ocf.berkeley.edu/debian buster InRelease: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 04EE7237B7D453EC NO_PUBKEY 648ACFD622F3D138 NO_PUBKEY DCC9EFBF77E11517
E: The repository 'mirrors.ocf.berkeley.edu/debian buster InRelease' is not signed.
N: Updating from such a repository can't be done securely, and is therefore disabled by default.
N: See apt-secure( manpage for repository creation and user configuration details.
Warning, command failed: apt-get update
Couldn't detect the appropriate kernel headers package!