Ok, I took a device with stock firmware, downloaded openwrt-ramips-mt7621-dlink_covr-x1860-a1-squashfs-factory.bin. OpenWrt is up and running.
Started recovery mode, uploaded COVR-X1860_RevA_Firmware_102b01_decrypted.bin: Stock firmware is running again.
Information about COVR-X1860_RevA_Firmware_102b01_decrypted.bin:
- Size: 16384352 Bytes
- SHA256: db053c31bdfdd75fc7022f997347224b9d8a61b6fa8ab42a5930f89797065c9a