Not sure whether board specific but might be interesting for developers which board revision Revisions - Turris Documentation your node is.
It appears to be a bug in the image (probably compiling) [U-Boot 2019.07 | rescue-image] 5-LED & 7-LED rescues fail (#602) · Issues · Turris / Turris OS / Turris OS packages · GitLab
Since the repo only offers the latest compiled image but no version tree you probably should contact support Support - Turris Documentation
Alternatively find another initram image for armv7l that can be booted from USB and has TFTF or ssh support build in.
Did you try different USB media and/or filesystems? Mishaps in that regard are not an uncommon occurrence
Output from the console should provide a hint, e.g. whether the USB drive or FS is not recognised or the medkit image is not found.