Ignore the rail planner app, it does not perform a check with the live reservations database.

The b-europe site gives the correct current availability

Unfortunately there is no availability on 22 July to Amsterdam.

There is to Brussels on just the 09.01 departure from London on that day.

If you want to use that, booking soon would be advised.

Thank you! just checked 1 seat left, I need 2 seats, do they release last minute or highly unlikely?

For ICE and other countries (Poland/Austria), which website I should use to check availability?

The Eurostar pass availability is linked to the regular fare brackets, once the last group Interrail passes are offered with is exhausted the public rate is on a higher bracket and stays that way until the train is sold out, so pass fares don’t reappear.

People traveling last minute are usually urgent travelers so like the airlines E* know they will more often than not suck it up and pay the highest fares so no last minute bargains.

If you are not in a rush for that journey a non-flying alternative is using the ferry from Harwich to Hook of Holland, this site explains that trip in detail, as well as most of Europe by rail.

https://www.seat61.com/Netherlands.htm

For general timetable and fare searches the German Rail website https://www.bahn.com/en

and Austrian https://www.oebb.at/en/ are good for much of Europe.

Here is information about how to make reservations for many different countries. 

The advice from the experienced travellers in the community is to use other ways to make reservations than the Interrail/Eurail website. You can look at the guide in the link:

https://community.eurail.com/train-connections-reservations-47/how-to-get-reservations-105

If you, after having looked at the guide, have questions about how to make specific reservation, please give your travel details (departure date, time and route) preferably in a new post, and you will get advice. 

Thanks! after reading the paper pass cover number story I think I will give up on this, my travel is less than 1 month away, I email Eurail same question has been one week but hasn’t got any reply, it leads me to no faith to their customer service when I will get a reply https://eurail.zendesk.com/hc/en-001/requests/new?ticket_form_id=360000353277

Again someone who did not read any info before and expects US-style sunny&helpful fone-it will not work-overworked and understaffed-most of the info-all newbees all ask the very same-is handled by a few regular contributors with many years of travel-knowhow. Even when travel with a pass you are also supposed to be able to fetch for yourself.