How to remove overlay partition?

Hi guys.
My ZyXEL NBG7815 (Armor G5) has 4 GB Flash but for installing packages available only 44 Mb. Flash is mmcblk, not mtd. Тhe Extroot coniguration says:

Note that OpenWrt is known to ignore the fstab configuration on devices without overlay partition in /proc/mtd . You can work around the issue by using / for the mount point on ROMs without overlay partition at all.

Tthis is my case.
Please help me understand how to mount on ROMs without overlay partition?

That's the result of the OEM partitioning and not going to change. The overlay will also always be there.

Does this mean that the remark in the Extroot about mount without overlay is wrong?