I had a server running on windows 10 through AT&T, but I have since moved and switched to TDS. I have completely removed my agent and tunnel, re-downloaded the agent and set it up from the beginning. I used my servers local IP address (and even put the IP on a port forward for 25565 in and out though playit doesn’t need a port forward).
No matter what I do, the web portal shows that there is no handshake between my network and playit servers. My local network has a GPON/Modem that has Routing set to bridge, and an AP/Router combo. It seems like a network issue for sure, but I am unsure what I would even ask my ISP to help fix to remove this issue.
As most network people will chime in right off the bat, it’s probably your DNS. 
Do you know what DNS provider you’re using? Have you tried different DNS serivice?google (8.8.8.8), cloudflare (1.1.1.1), quad9 (9.9.9.9).
I’ve had different modems try to inject their own DNS, which was supper annoying. You may see if swapping the DNS service on the computer you’re using it on may help.
I had my router set to OpenDNS, but I have changed it to 9.9.9.9 with secondary 1.1.1.2
On my server, I did a DNS flush and set it to the DNS listed above.
I am still not able to get a connection to the server over PlayIt.gg IP. I can play on the server through its local IP, but that is it.
are you running this on your personal computer or dedicated machine running something like crafty or something? have you confirmed the computer is using the DNS being offered or maybe using something saved?
have you tried a different install method? like if you were on linux (or windows for that matter) have you tried using the docker version?
Have you tried a different computer to try to install it on?
If you can try to install it on another computer with WIFI, can you hook up to a hotspot and remove your home internet entirely?
When I was having issues in the past, my agents could connect just fine, but my home internet DNS wasn’t allowing me to connect if i was going out to the internet and then back to my server (I wasn’t trying to connect locally, I was testing things and wanted to test the public internet connecting).
the weird part of the whole thing is this is essentially a VPN tunnel from their server to your machine. Not unless your ISP is blindly blocking VPN stuff, there’s nothing on the ISP side that should matter. Like you aren’t doing any port forwarding at your modem, it’s an encrypted tunnel from their server to your computer.
I’m not poo pooing playits service, i think its amazing for a free service. But I migrated to using a VPS with a public IP so I could essentially do the same thing they’re offering, but on my own. it clearly required a lot more work on my end to set it up, but in the end the added stability is what kept me on my path.