This is not really a computer issue, but one related to cell phones. But since being able to access the Internet on your computer when your ISP is not available is so useful, I post my tip here.
There are three obvious situations when your computer cannot access your ISP: when you are not physically near your computer's modem/router or it is not working, or your ISP is experiencing technical problems.
Most Android phone have two options that can help in this case. One is "portable wifi hotspot" and the other is "USB tethering". A third option "bluetooth tethering", also might help, but I have never tried that.
Whenever I followed the directions on my cell phone for setting up a portable wifi hotspot, my Linux OS always recognized it and was able to connect to it. However, I still could not use the Internet because there was no domain-name service (DNS). USB tethering, which previously showed connectivity and DNS, recently stopped providing the latter for me. The DNS problem, seems to be related to the data plan your mobile provider is giving you, although it may also have something to do with your Android or cellphone version.
Normally, your mobile provider gives you a SIM card, which contains the information necessary to set up an APN on your cell phone. If this process does not work as expected, it is possible to set up a working APN for your provider using the menus and options of Android (Options->Connections->Mobile Network->Access Point Names). You can find the necessary values for the entries, either from your provider or by doing an Internet search. A key entry with regard to DNS working, is "APN Type". The entry can have multiple values, separated by commas, but not spaces. My mobile provider recommends "default,supl,agps,fota" without the quotation marks, and until recently, that was enough to get DNS working with USB tethering but not with portable wifi hotspot. Currently, if I change the entry to "default,supl,agps,fota,dun", again without quotation marks, DNS works for both USB tethering and portable wifi hotspot, and I can use either to surf the Internet on my Linux computer. The speed is that of my mobile provider, rather than the faster speed of my ISP using a high-speed modem/router, but is quite adequate for many applications. I assume you will have to pay your mobile carrier according to your contract and the increased data volume.
In today's world, we have dozens of passwords to remember, and it is comforting that the leading manufacturer of modem/routers uses the easy-to-remember default admin/admin as the initial login credentials. Do not be fooled into thinking that because your computer and modem/router are physically secure (i.e., no one has physical access to them), that you don't need a good strong password to protect them from hackers.
Don't take my word for this. Go to any of those Internet sites that says they will tell you your public IP address, such as www.iplocation.net. If it says your IP4 address is "W.X.Y.Z", type in "http://W.X.Y.Z" in any browser, and the browser will navigate to your modem/router's login page. This is why your computer and modem/router need good strong passwords.