Issue with email from mail server over Telenet wifi

[RobM]

Please forgive my use of English here. Please do feel free to reply in Dutch though.

2 day issue. No mail in or out from Bluehost. My host for many years. This isn't an issue with Telenet mail, I don't use a Telenet mail account. 
Desktop MacMini M2
Macbook pro
Iphone.
Apple Mail software 

All were working on the standard Telenet wifi with no issues. Since 2 days, errors that call for password entry yet cannot contact server.
Iphone connects to mail server on 4G
Laptop connects to server on my modem's telenetwifree channel but not on the main house wifi. 
Desktop the same.
Desktop and laptop can be tethered to the iPhone 4G and all works fine.
LAN also does not work to connect to that host.
Mail server passwords changed, detailed check of settings. Nothing was changed on all 3 devices before the errors started.

I moved the laptop to anther address with Telenet and it worked on wifi

I have spent many hours on Twitter chat and direct support chat with Bluehost. They have looked everywhere there. They can't find any issues. I can access all mails via my Cpanel webmail. 

I've tried asking Telenet support on Twitter but they don't even think there is an issue and suggested I ask on here. I'm not at all impressed with their support when after all my testing the only issue is singled down to attempting to connect to my host mail accounts from my home modem. The white, newer type.

Any help welcome. It's making me nuts. many thanks. 

Bedankt, Rob

[kristofDW]

@RobM Could it be that there is a firewall in between that is blocking your mail traffic?

Since it works on another connection and your way of using mail is the same.

Be sure to check your firewall on your PC and laptop.

It is possible that e-mail traffic is blocked on certain internet connections.

[RobM]

@kristofDW I'm having the problem on a Mac Mini M2, MacBook pro an an iPhone. 
I've checked the OS firewall is off on the 2 computers and it's the same issue.  I'm not aware of any other firewall between my and the modem. 

Telenet support themselves, via Twitter have shrugged it off as 'not our problem' The most simple way to rule that out would be for an engineer to come and replace the modem I think.
My host, Bluehost has run checks on blacklists and the server I'm on there isn't listed.  It's strange isn't it. All was fin cup until the 8th May. All devices worked fine. 
Unfortunately I don't have a PC here to set up.