Seem you do the right way.

Which firmware version are you using for? Could you please upgrade to v3.009 and try those commands?

opkg update
opkg install kmod-i2c-gpio-custom
opkg install kmod-rtc-sd2068
insmod i2c-gpio-custom bus0=0,13,17
echo "i2c-gpio-custom bus0=0,13,17" > /etc/modules.d/i2c-gpio-custom
echo sd2068 0x32 > /sys/bus/i2c/devices/i2c-0/new_device
reboot

Here is all my steps.

root@GL-MIFI:/# opkg update
Downloading http://download.gl-inet.com/releases/kmod-3.0/ar71xx/generic/Packages.gz
Updated list of available packages in /var/opkg-lists/glinet_core
Downloading http://download.gl-inet.com/releases/packages-3.x/ar71xx/base/Packages.gz
Updated list of available packages in /var/opkg-lists/glinet_base
Downloading http://download.gl-inet.com/releases/packages-3.x/ar71xx/gli_pub/Packages.gz
Updated list of available packages in /var/opkg-lists/glinet_gli_pub
Downloading http://download.gl-inet.com/releases/packages-3.x/ar71xx/packages/Packages.gz
Updated list of available packages in /var/opkg-lists/glinet_packages
Downloading http://download.gl-inet.com/releases/packages-3.x/ar71xx/luci/Packages.gz
Updated list of available packages in /var/opkg-lists/glinet_luci
Downloading http://download.gl-inet.com/releases/packages-3.x/ar71xx/routing/Packages.gz
Updated list of available packages in /var/opkg-lists/glinet_routing
Downloading http://download.gl-inet.com/releases/packages-3.x/ar71xx/telephony/Packages.gz
Updated list of available packages in /var/opkg-lists/glinet_telephony
Downloading http://download.gl-inet.com/releases/packages-3.x/ar71xx/glinet/Packages.gz
Updated list of available packages in /var/opkg-lists/glinet_glinet
root@GL-MIFI:/# opkg install kmod-i2c-gpio-custom
Installing kmod-i2c-gpio-custom (4.9.120-2) to root...
Downloading http://download.gl-inet.com/releases/kmod-3.0/ar71xx/generic/kmod-i2c-gpio-custom_4.9.120-2_mips_24kc.ipk
Installing kmod-i2c-core (4.9.120-1) to root...
Downloading http://download.gl-inet.com/releases/kmod-3.0/ar71xx/generic/kmod-i2c-core_4.9.120-1_mips_24kc.ipk
Installing kmod-i2c-algo-bit (4.9.120-1) to root...
Downloading http://download.gl-inet.com/releases/kmod-3.0/ar71xx/generic/kmod-i2c-algo-bit_4.9.120-1_mips_24kc.ipk
Installing kmod-i2c-gpio (4.9.120-1) to root...
Downloading http://download.gl-inet.com/releases/kmod-3.0/ar71xx/generic/kmod-i2c-gpio_4.9.120-1_mips_24kc.ipk
Configuring kmod-i2c-core.
[  336.165751] kmodloader: loading kernel modules from /etc/modules.d/*
[  336.183076] i2c /dev entries driver
[  336.194914] u32 classifier
[  336.196167]     input device check on
[  336.199811]     Actions configured
[  336.221557] kmodloader: done loading kernel modules from /etc/modules.d/*
Configuring kmod-i2c-algo-bit.
[  336.423351] kmodloader: loading kernel modules from /etc/modules.d/*
[  336.443092] kmodloader: done loading kernel modules from /etc/modules.d/*
Configuring kmod-i2c-gpio.
[  336.642956] kmodloader: loading kernel modules from /etc/modules.d/*
[  336.657537] kmodloader: done loading kernel modules from /etc/modules.d/*
Configuring kmod-i2c-gpio-custom.
root@GL-MIFI:/# opkg install kmod-rtc-sd2068
Installing kmod-rtc-sd2068 (4.9.120-1) to root...
Downloading http://download.gl-inet.com/releases/kmod-3.0/ar71xx/generic/kmod-rtc-sd2068_4.9.120-1_mips_24kc.ipk
Configuring kmod-rtc-sd2068.
[  349.575436] kmodloader: loading kernel modules from /etc/modules.d/*
[  349.593743] kmodloader: done loading kernel modules from /etc/modules.d/*
root@GL-MIFI:/# insmod i2c-gpio-custom bus0=0,13,17
[  352.360042] Custom GPIO-based I2C driver version 0.1.1
[  352.364370] i2c-gpio i2c-gpio.0: using pins 13 (SDA) and 17 (SCL)
root@GL-MIFI:/# echo sd2068 0x32 > /sys/bus/i2c/devices/i2c-0/new_device
[  360.536561] rtc-sd2068 0-0032: rtc core: registered sd2068 as rtc0
[  360.547823] i2c i2c-0: new_device: Instantiated device sd2068 at 0x32
root@GL-MIFI:/# 
root@GL-MIFI:/# 
root@GL-MIFI:/# 
root@GL-MIFI:/# hwclock -w
root@GL-MIFI:/# hwclock -r
Wed Feb 27 04:18:51 2019  0.000000 seconds