A demo script to detect by url if a specific interface is up.

RouterOS-Detect-internet-connection.rsc

{
	:do {
		:local srcInterface1 "ether1-WAN1";
		:local srcInterface2 "ether2-WAN2";
		:local srcInterface3 "ether3-LTE";
		:local srcInterface4 "pppoe-WAN1";
		:local srcInterface5 "pppoe-WAN2";
		:local srcInterface6 "pppoe-LTE1";
		:local flagInterface "";
		
		:set flagInterface $flagInterface3;
		
		
		:local srcAddressCidr [/ip/address/get [find where interface=ether3-LTE ] address ];
		:local srcAddress "";
		:local urlString1 "https://ipconfig.io/country";
		:local urlString2 "http://www.msftncsi.com/ncsi.txt";
		:local urlString3 "https://connectivitycheck.gstatic.com";
		:local urlString4 "https://www.apple.com/library/test/success.html";
		
		:local result "";
		:local flagInterface1 "lte1-flag1";
		:local flagInterface2 "lte1-flag2";
		:local flagInterface3 "lte1-flag3";
		:local flagInterface "";
		:set flagInterface $flagInterface1;

	:do {
		if ([/interface/bridge/print count-only where name=$flagInterface ] = 0) do={
			[/interface/bridge/add name=$flagInterface disabled=yes];
		}
	} on-error={
		:log info "Error creating the flag interface";
	}
		:local currentFlagDisabledState [/interface/bridge/get  [find where name=$flagInterface ] value-name=disabled ];


		:for i from=( [:len $srcAddressCidr] - 1) to=0 do={
			:if ( [:pick $srcAddressCidr $i] = "/") do={
				:set srcAddress [:pick $srcAddressCidr 0 $i];
			}
		}
		
		:do {
			:set result [/tool/fetch src-address=$srcAddress url=$urlString1 as-value output=none];
			if ($currentFlagDisabledState = true) do={
				[/interface/bridge/set [find where name=$flagInterface ] disabled=no ];
			}
		} on-error={
			:log info ("Error downloading url: " . $urlString1);
			if ($currentFlagDisabledState = false) do={
				[/interface/bridge/set [find where name=$flagInterface ] disabled=yes];
			}
		}
		:log info "success";
		
	} on-error={
		:log info "Runtime Error";
	}

}