Is this the ISP's DNS at fault?

Do you have access to the settings for your ISP router ? If so you should be able to set your DNS address manually ...

If not you can go into the DNS settings in the network setups for each of your devices and set the primary and secondary DNS addresses- the system is setup to use the first DNS found in the order of ...

Device set DNS Primary, Secondary.
Router Set DNS, Primary, Secondary.
ISP Default DNS Primary, Secondary

Personally I use Cloudflare for Primary 1.1.1.1 & Google for Secondary 8.8.8.8. [although its often setup to use the Cloudflare or Google primary as primary and retain the ISP primary as secondary DNS if your ISP uses private dns for its own services ,(not common)]

And rarely have any issues ( some of the more secure sites do are not recorded in DNS databases)

One way to find out what is going is to open a command prompt or in powershell and use nslookup

ping is not the best way to figure out what is going on some sites disable answering ping or if the server is busy it will just drop the request

Code:

PS C:\Users\mydig> nslookup whatever.com

Server:  pi11.pi11
Address:  192.168.200.11                                                                                                                                                                 Non-authoritative answer:
Name:    whatever.com
Address:  184.168.131.241

Lets break that down so you understand what the above means

this is the name of the DNS server that you are currently using (yes I run my own DNS server on my local network)

Code:

Server:  pi11.pi11

This is the IP of the DNS server you are currently using

Code:

Address:  192.168.200.11

The is the URL that you are interested in.

Code:

Name:    whatever.com

This is the IP of the URL that you are interested in

Code:

Address:  184.168.131.241

if the site is not found it will show something like this

Code:

*** pi11.pi11 can't find bbbbbbbbmkjajsagh.com: Non-existent domain

If your search finds nothing or you think its your ISP's fault you can direct your query to another server with nslookup like this.

Code:

nslookup whatever.com 1.1.1.1

You could also try tracert command to see where it is getting hung up
tracert uses the ping command and if the site your interested in blocks ping it might not be so helpful

Code:

tracert whatever.com

secondary using ISP's original

If your ISP is blocking the web site you want to view then it will probably still be blocked.

Primary and secondary DNS server's may not work in the way most people expect, Windows and most other operating systems will use the fastest server whether it's primary or secondary and in your case I can almost guarantee you that it will use your ISP because it will be the fastest.

EDIT:
If you want to find the best (fastest) try Domain Name Speed Benchmark

GRC's | DNS Nameserver Performance Benchmark

download (it's at the bottom of the page)
https://www.grc.com/files/DNSBench.exe

The Cloudflare and Google DNS servers are on a higher speed level, [ No 1 & 2 in independent industry tests], to any of the ISP DNS Servers

Speed is not the only parameter that affects the choice of Server

With Cloudflare and Google, Plus the New 9.9.9.9 service, there are multiple server locations so there should be one as close, or closer, to the user than their ISPs server. and they also run directly on the Main Internet backbone so have faster access to less popular addresses when fetching from the official Nameservers, [many are actually operated by Cloudflare or Google]

There is also the availability factor to take into account, The top DNS Servers will be available 24/7/365 and have sufficient capacity to serve many more users than they do now, something that a smaller ISP system cannot guarantee - It could be that the failure, or delay, of the ISPs server to return the site requested could simply be because they are overloaded due to the limit of their bandwidth

One other thing to note with Cloudflare and Google DNS servers is that the results are generally fresher, in my experience ISP DNS can be stale and not as quick to update.

Not sure if your interest but Cloudflare has some additional servers for blocking malware and they also have servers to block porn.
Introducing 1.1.1.1 for Families


Malware Blocking Only
Primary DNS: 1.1.1.2
Secondary DNS: 1.0.0.2

Malware and Adult Content
Primary DNS: 1.1.1.3
Secondary DNS: 1.0.0.3

For IPv6 use:
# you do not have to use the below unless your running a IPv6 network only, If you have a IPv4 address use the above.

Malware Blocking Only
Primary DNS: 2606:4700:4700::1112
Secondary DNS: 2606:4700:4700::1002

Malware and Adult Content
Primary DNS: 2606:4700:4700::1113
Secondary DNS: 2606:4700:4700::1003