SSH to your Synology Diskstation as “root”.
The “Autoblock list” is located at /etc/synoautoblock.db
This is a SQLLite database, so don’t try to edit it with VI.

sqlite3 synoautoblock.db
.header on
select * from AutoBlockIP;
sqlite> select * from AutoBlockIP;

IP RecordTime ExpireTime Deny IPStd
————– ———- ———- ———- —————————————
blocked IP xxx.xxx.xxx.xxx 0 1 0000:0000:0000:0000:0000:FGGG:C3A8:B415

delete from AutoBlockIP where IP = “xxx.xxx.xxx.xxx”;
.exit