does this help by using the “reserved” partition?
root@swi-mdm9x28-wp:~# more /proc/mtd
dev: size erasesize name
mtd0: 00280000 00040000 "sbl"
mtd1: 00280000 00040000 "mibib"
mtd2: 00680000 00040000 "backup"
mtd3: 00100000 00040000 "security"
mtd4: 00200000 00040000 "persist"
mtd5: 01180000 00040000 "efs2"
mtd6: 04f40000 00040000 "swifota"
mtd7: 00180000 00040000 "tz"
mtd8: 000c0000 00040000 "devcfg"
mtd9: 000c0000 00040000 "rpm"
mtd10: 02000000 00040000 "modem"
mtd11: 00100000 00040000 "aboot"
mtd12: 00f00000 00040000 "boot"
mtd13: 01e00000 00040000 "system"
mtd14: 008c0000 00040000 "lefwkro"
mtd15: 01900000 00040000 "swirw"
mtd16: 08300000 00040000 "userapp"
mtd17: 03680000 00040000 "reserved"
mtd18: 02180000 00040000 "slot_2"
mtd19: 02180000 00040000 "slot_1"
mtd20: 02180000 00040000 "slot_0"
root@swi-mdm9x28-wp:~# ubiformat /dev/mtd17 -y
ubiformat: mtd17 (nand), size 57147392 bytes (54.5 MiB), 218 eraseblocks of 262144 bytes (256.0 KiB), min. I/O size 4096 bytes
libscan: scanning eraseblock 217 -- 100 % complete
ubiformat: 218 eraseblocks are supposedly empty
ubiformat: formatting eraseblock 217 -- 100 % complete
root@swi-mdm9x28-wp:~# ls /dev/ubi*
/dev/ubi0 /dev/ubi1_0 /dev/ubi4 /dev/ubiblock1_0
/dev/ubi0_0 /dev/ubi2 /dev/ubi4_0 /dev/ubiblock2_0
/dev/ubi0_1 /dev/ubi2_0 /dev/ubi_ctrl
/dev/ubi0_2 /dev/ubi3 /dev/ubiblock0_0
/dev/ubi1 /dev/ubi3_0 /dev/ubiblock0_1
root@swi-mdm9x28-wp:~# ls /dev/ubi* -l
crw-rw---- 1 root root 238, 0 Jan 1 1970 /dev/ubi0
crw-rw---- 1 root root 238, 1 Jan 1 1970 /dev/ubi0_0
crw-rw---- 1 root root 238, 2 Jan 1 1970 /dev/ubi0_1
crw-rw---- 1 root root 238, 3 Jan 1 1970 /dev/ubi0_2
crw-rw---- 1 root root 234, 0 Jan 1 1970 /dev/ubi1
crw-rw---- 1 root root 234, 1 Jan 1 1970 /dev/ubi1_0
crw-rw---- 1 root root 236, 0 Jan 1 1970 /dev/ubi2
crw-rw---- 1 root root 236, 1 Jan 1 1970 /dev/ubi2_0
crw-rw---- 1 root root 237, 0 Jan 1 1970 /dev/ubi3
crw-rw---- 1 root root 237, 1 Jan 1 1970 /dev/ubi3_0
crw-rw---- 1 root root 235, 0 Jan 1 1970 /dev/ubi4
crw-rw---- 1 root root 235, 1 Jan 1 1970 /dev/ubi4_0
crw-rw---- 1 root root 10, 57 Jan 1 1970 /dev/ubi_ctrl
brw-rw---- 1 root root 253, 0 Jan 1 1970 /dev/ubiblock0_0
brw-rw---- 1 root root 253, 1 Jan 1 1970 /dev/ubiblock0_1
brw-rw---- 1 root root 253, 3 Jan 1 1970 /dev/ubiblock1_0
brw-rw---- 1 root root 253, 2 Jan 1 1970 /dev/ubiblock2_0
root@swi-mdm9x28-wp:~# ubiattach -m 17 -d 7
UBI device number 7, total 218 LEBs (55361536 bytes, 52.7 MiB), available 174 LEBs (44187648 bytes, 42.1 MiB), LEB size 253952 bytes (248.0 KiB)
root@swi-mdm9x28-wp:~# ubimkvol /dev/ubi7 -N test -s 42MiB
Volume ID 0, size 174 LEBs (44187648 bytes, 42.1 MiB), LEB size 253952 bytes (248.0 KiB), dynamic, name "test", alignment 1
root@swi-mdm9x28-wp:~# mkdir -p /tmp/customer
root@swi-mdm9x28-wp:~# mount -t ubifs /dev/ubi7_0 /tmp/customer
root@swi-mdm9x28-wp:~# ls /tmp/customer/
root@swi-mdm9x28-wp:~# echo safsdfasdf > /tmp/customer/test.txt
root@swi-mdm9x28-wp:~# cm info
Device: WP7608
IMEI: 352913090501857
IMEISV: 6
FSN: XG207430721010
Firmware Version: SWI9X07Y_02.28.03.03 000000 jenkins 2019/05/21 03:33:04
Bootloader Version: SWI9X07Y_02.28.03.03 000000 jenkins 2019/05/21 03:33:04
MCU Version: 002.011
PRI Part Number (PN): 9908665
PRI Revision: 001.004
Carrier PRI Name: GENERIC
Carrier PRI Revision: 002.068_003
SKU: 1104194
Last Reset Cause: Power Down
Resets Count: Expected: 15 Unexpected: 0
root@swi-mdm9x28-wp:~# cat /tmp/customer/test.txt
safsdfasdf
// power cycle the device
root@swi-mdm9x28-wp:~# mkdir -p /tmp/customer
root@swi-mdm9x28-wp:~# ubiattach -m 17 -d 7
UBI device number 7, total 218 LEBs (55361536 bytes, 52.7 MiB), available 0 LEBs (0 bytes), LEB size 253952 bytes (248.0 KiB)
root@swi-mdm9x28-wp:~# mount -t ubifs /dev/ubi7_0 /tmp/customer
root@swi-mdm9x28-wp:~# cat /tmp/customer/test.txt
safsdfasdf
root@swi-mdm9x28-wp:~#