opkg update
opkg install resize2fs

root@OpenWrt:~# df
Filesystem           1K-blocks      Used Available Use% Mounted on
udev                       512         0       512   0% /dev
/dev/mmcblk0p2          197209    190164      2949  98% /
tmpfs                   945248      1616    943632   0% /tmp
tmpfs                      512         0       512   0% /dev
/dev/mmcblk0p1          130798     22210    108588  17% /mnt/mmcblk0p1
/dev/mmcblk0p2          197209    190164      2949  98% /mnt/mmcblk0p2

fdisk /dev/mmcblk0
fdisk /dev/sda

root@OpenWrt:~# fdisk /dev/mmcblk0

Welcome to fdisk (util-linux 2.33).
Changes will remain in memory only, until you decide to write them.
Be careful before using the write command.


Command (m for help): m

Help:

  DOS (MBR)
   a   toggle a bootable flag
   b   edit nested BSD disklabel
   c   toggle the dos compatibility flag

  Generic
   d   delete a partition
   F   list free unpartitioned space
   l   list known partition types
   n   add a new partition
   p   print the partition table
   t   change a partition type
   v   verify the partition table
   i   print information about a partition

  Misc
   m   print this menu
   u   change display/entry units
   x   extra functionality (experts only)

  Script
   I   load disk layout from sfdisk script file
   O   dump disk layout to sfdisk script file

  Save & Exit
   w   write table to disk and exit
   q   quit without saving changes

  Create a new label
   g   create a new empty GPT partition table
   G   create a new empty SGI (IRIX) partition table
   o   create a new empty DOS partition table
   s   create a new empty Sun partition table


Command (m for help): d
Partition number (1,2, default 2): 

Partition 2 has been deleted.

Command (m for help): n
Partition type
   p   primary (1 primary, 0 extended, 3 free)
   e   extended (container for logical partitions)
Select (default p): 

Using default response p.
Partition number (2-4, default 2): 
First sector (264192-30547967, default 264192): 
Last sector, +/-sectors or +/-size{K,M,G,T,P} (264192-30547967, default 30547967): 

Created a new partition 2 of type 'Linux' and of size 14.5 GiB.
Partition #2 contains a ext4 signature.

Do you want to remove the signature? [Y]es/[N]o: N

Command (m for help): w

The partition table has been altered.
Syncing disks.

resize2fs /dev/mmcblk0p2
resize2fs /dev/sda2
btrfs filesystem resize max /

root@OpenWrt:~# resize2fs /dev/mmcblk0p2
resize2fs 1.44.5 (15-Dec-2018)
Filesystem at /dev/mmcblk0p2 is mounted on /; on-line resizing required
old_desc_blocks = 1, new_desc_blocks = 58
The filesystem on /dev/mmcblk0p2 is now 15141888 (1k) blocks long.

reboot

root@OpenWrt:~# df
Filesystem           1K-blocks      Used Available Use% Mounted on
udev                       512         0       512   0% /dev
/dev/mmcblk0p2        14670958    191357  14475505   1% /
tmpfs                   945248       924    944324   0% /tmp
tmpfs                      512         0       512   0% /dev
/dev/mmcblk0p1          130798     22210    108588  17% /mnt/mmcblk0p1

btrfs filesystem resize max /