Can an ISP block certain ports of websites and servers?

For some time the error occurs more and more that I can no longer log into PSN with my PS4. However, I can change this to my mobile hotspot, for example. I would like to use my Fritzbox again, I can no longer connect to the Internet according to PS4, but Netflix I can continue to look and that without alleged Internet connection. This error occurs only in phases. For example, yesterday I was able to play a few rounds online for no reason. Today, this is no longer possible. In addition, if this error occurs, the website of the PSN's no longer loads. I already contacted the PSN. But now that I need the support again, the pages do not load anymore from the PSN.

Therefore, I ask myself the question, whether the PSN may be simply overloaded in this area or it may be related to my Internet provider blocking certain ports and servers. Is it on the hardware of my PS4 or on the router?

Hello - is the PSN page (exactly itself link) on other devices, just blocked as your smartphone?

Yes exactly. Any pages of the Playstation support do not load. At least if I use my WLAN.

I do not think your provider would like to annoy you.
If possible, look into your Fritzbox. There should be settings can be made - may also be displayed traffic to the website. Ports can be set, etc. Since I unfortunately have no FritzBox, I can't describe you point-to-point instructions, so click around there times.

If the way is not over WLAN - but about LTE already, I think that it is s.der FritzBox. That's why my guess. Did you start this again?