Your problem is with the MTU setting for your DSL connection.
There's a MTU setting in Ubuntu's network configuration, and a WAN MTU setting in your router.
For DSL, a common MTU setting is 1492. Just go ahead and try this value first and see if your web sites are now accessible.
To determine the correct setting, start with all MTU settings = 1500 and VPN = off. (VPN requires different testing).
ping [-c count] [-M do] [-s packet_size] [host]
The options used are:
c count: number of times to ping
M hint: Select Path MTU Discovery strategy. may be either
do (prohibit fragmentation, even local one),
want (do PMTU discovery, fragment locally when packet size is large), or
dont (do not set DF flag).
s packet_size: Specifies the number of data bytes to be sent.
You should always start at 1472 and work your way down by 10 each time. Once you get a reply, go up by 1 until you get a fragmented packet. Take that value (last good value) and add 28 to the value to account for the various TCP/IP headers. Eg. let's say that 1452 was the proper packet size (where you first got an ICMP reply to your ping). The actual MTU size would be 1480, which is the optimum for the network we're working with.
ping -c 4 -M do -s 1472 126.96.36.199 # this will probably show fragmentation
ping -c 4 -M do -s 1462 188.8.131.52 # may show fragmentation
ping -c 4 -M do -s 1452 184.108.40.206 # no fragmentation?
ping -c 4 -M do -s 1453 220.127.116.11 # still no fragmentation?
reference: How to determine the proper MTU size with ICMP pings