last week i have successfully connected 3g modem with PTCL modem router, i will guide you how you can do this easily.
i have used the follwing model of PTCL modem router for this purpose which is evo chargi compatible.

Use Any 3G USB Modem With PTCL Modem Router

remember you need a 3g USB modem with builtin wifi hotspot, otherwise it may not work,
the reason why you need hotspot enabled 3g USB modem is, that PTCL router itself cannot install drivers and don’t have built in app to connect 3g USB to internet. it will only forward dialing command to USB modem and modem have to connect by itself.

Lets come to configuration guide.

1. Buy any wifi enabled 3g Modem from Market (mine was chinese brand HSPA+) cheapest i found.
2. connect it to USB port of PTCL router, if it doesn’t fit properly because routers design, then use USB extension cable as i used, you can see in the image below

Use Any 3G USB Modem With PTCL Modem Router

3. once 3gUSB modem connected properly turn on PTCL router. you will see 2 wifi networks, one for PTCL router and 2nd for 3g Modem . as you can see in the image below my usb modem network name is “3G ufi_9FC” most usb modems have default wifi password: 1234567890 or it will be written on modem box. after entering proper password you will be connected.

Use Any 3G USB Modem With PTCL Modem Router

4. after you are connected , open your browser and access the admin panel of your 3g USB modem,
my modems access IP was 192.168.100.1, yours could be different check manual inside 3g modem box.

Use Any 3G USB Modem With PTCL Modem Router

5. By default username & password is admin , admin, but if it doesn’t work then look in your manual.
6. after successful login you will see similar settings screen. as shown in image below.

Use Any 3G USB Modem With PTCL Modem Router

7. you will see admin panel menu on this page, you need to click on 3g Settings  icon and then click on connection mode and chose manual dial-up. this is most important step. see in image below.

Use Any 3G USB Modem With PTCL Modem Router

(Note: if you want to use 3g USB modem using using USB power adapter then you must change to automatic dialing mode)

8. Normally USB modem will pickup your Access Point settings, but incase it doesn’t receive it automatically, you can add APN settings by clicking on 3g settings menu. and on next page you will see option to add new APN. my modem automatically saved telenor settings.

Use Any 3G USB Modem With PTCL Modem Router

9. you must change your 3g USB modem wifi password so that no one can access your internet, for this you can click on wifi settings button and chose WiFi security, on next page you will be able to change your 3G USB modem wifi password, see image below.

Use Any 3G USB Modem With PTCL Modem Router

10. next step is to configure your PTCL modem for this 3G USB modem. now you have to connect to your PTCL router wifi network or connect using LAN cable. and open the admin panel by entering access IP that could be 192.168.10.1 or 192.168.1.1.

11. Once you are logged in you need to goto advanced settings, in my PTCL router advanced setting button was in right side top corner, as you can see in image below.

Use Any 3G USB Modem With PTCL Modem Router

12. in advanced settings , Go to USB-3G/4G setting menu ,and enter you APN settings same as you saved in your 3g USB modem, as you can see in image below.

Use Any 3G USB Modem With PTCL Modem Router

13. Apply/Save these settings, and your 3g internet will be connected, now you can share your 3g internet with PTCL router just like ADSL connection. you can also check status of connection by entering admin panel of PTCL router and go to the end of page you will see 3G connection status.

Use Any 3G USB Modem With PTCL Modem Router

This whole process has been elaborated in the following videos, you can watch these videos for more help