@Help2023  I think, I should enter 192.168.1.9 in the destination IP Address as my extender's IP address is that as you can see in the first image of my last reply. Don't you think so? And in two of the sections, where you said you're not sure, I'm also not sure what to put in those boxes. One of them (the last one) has a drop down menu with two options which I can try one after one but still there left one box where I need to put something but I don't know what to enter there (it may be 0 or just leave it blank, don't know). So, I'm thinking that I should call my ISP for further assistance. 

I also wanted to ask you, is the 2nd image (Routing section in network settings) the right place for changing the extender's IP into a static one?

  @Help2023 I tried your this setting but it didn't work. Thanks for your help. 

  @Help2023 Yes, I did change DHCP setting from auto to enable and put the entries just as you did. But unfortunately, nothing fruitful happened. It (the extender) was working very smooth with the last ISP's router, but now when I changed the ISP, it's quite a pain in the a** (LOL). The new ISP's router is dual band (2.4G & 5G), may be this is the reason behind all the problem. Not sure though. I need the extender for better coverage across all the house, and for the 2.4G Wi-Fi camera where my router can't reach. But nothing seems to work properly. 

Thanks for your kind help, anyway. Have a good day / night ahead.

  @Help2023 Hello Help2023! I think I did it. Gave my extender an permanent static IP and changed the settings in the Tether app just as you shown. It's working, for now. But, I've a 2.4 Ghz Wi-Fi dongle too for my PC. So, I wanna ask do you have a Wi-Fi dongle and did you try to connect your PC via Extender's Wi-Fi? As when I try to connect my PC to Extender's Wi-Fi, it, at first, don't connect but when it connects, it blocks again the Ethernet connection of the PC from main router and also don't connect via Wi-Fi of the main router, plus it also interrupts the main router's Wi-Fi in other mobile devices. 

Don't know why this is happening but still, if I don't connect my PC to the Extender's Wi-Fi, everything seems alright, thanks to you.   

I just hit the Forget this Network for Extender's Wi-Fi in the PC lest it shouldn't connect to it automatically when available.