Yes, 16 MB RAM is really little for 18.06.1.
And 4 MB flash is almost as troublesome as the RAM shortage.
(due to the tiny RAM amount, I am a bit surprised that even 17.01 works for you. Even that may reboot due to OOM oops, if you do anything memory intensive, like installing a large package)
Ps. Your devices wiki page ( https://openwrt.org/toh/hwdata/sagem/sagem_fast2604 contains a warning about 4/32 problems), but your device is actually even smaller 4/16, which should be non-existent nowadays. If I remember right, it was the 12.09 version (6 years ago) or 10.03 that was the last one recommended for 16 MB RAM devices...