Why some websites don't work properly?

I have Asus RT-AC87U router coected to inteet via Huawei E3372 USB modem.

• When coecting Huawei directly to my PC, everything works fine
• When coected via Asus router AND using VPN, everything works fine
• When coected via Asus router without VPN some websites don't load at all, while some load partially (and forever remain in loading state) and finally some websites work as expected.

Tracing route to acme.pl [xxx.yy.230.87]
over a maximum of 30 hops:

 1     2 ms     2 ms     2 ms  router.asus.com [192.168.1.1]                        
 2    28 ms    28 ms    28 ms  hi.link [192.168.8.1]                                
 3     *        *        *     Request timed out.                                   
 4    45 ms    44 ms    54 ms  10.220.215.34                                        
 5    42 ms    35 ms    46 ms  10.220.101.17                                        
 6    48 ms    41 ms    43 ms  wawbal-i1-BE6-301.plusnet.pl [212.2.99.241]          
 7    38 ms    40 ms    36 ms  wawbal-i2-BE12.plusnet.pl [212.2.102.210]            
 8    92 ms    78 ms    72 ms  blackhole.prolexic.com [80.81.193.70]                
 9    63 ms    79 ms    53 ms  unknown.prolexic.com [72.52.48.192]                  
10    75 ms    76 ms    77 ms  unknown.prolexic.com [72.52.48.197]                  
11     *        *        *     Request timed out.                                   
12    96 ms    94 ms    82 ms  unknown.prolexic.com [72.52.21.186]                  
13     *        *        *     Request timed out.                                   
14     *        *        *     Request timed out.                                   
15     *        *        *     Request timed out.                                   
16     *        *        *     Request timed out.                                   
17     *        *        *     Request timed out.                                   
18     *        *        *     Request timed out.                                   
19     *        *        *     Request timed out.                                   
20     *        *        *     Request timed out.                                   
21     *        *        *     Request timed out.                                   
22     *        *        *     Request timed out.                                   
23     *        *        *     Request timed out.                                   
24     *        *        *     Request timed out.                                   
25     *        *        *     Request timed out.                                   
26     *        *        *     Request timed out.                                   
27     *        *        *     Request timed out.                                   
28     *        *        *     Request timed out.                                   
29     *        *        *     Request timed out.                                   
30     *        *        *     Request timed out.                                   

Trace complete.

When doing tracert acme.com to the problematic website I noticed I'm getting curious domains - blackhole.prolexic.com and unknown.prolexic.com.

Could that be the cause? Are my packets lots in a blackhole and my router is forever repeating them in hopes of getting a response?

How can I fix that? Why am I getting blackholed?

The routers default USB MTU setting is 0 - I assume this means auto or unlimited. I guess that shouldn't be the problem, as MTU should be negotiated between hop destinations?

Also, why does it works via VPN, but not normally?

Another example:

• Superuser.com main website works fine
• My profile page does not load (unless via VPN)
• Question view page works fine
• Question edit page does not load (unless via VPN)

I think maybe some assets loaded from another domain are blocked?