One concern was that the 4G SIM card would support the required services eg mail server, web server, ssh access. The TPG ADSL service did not block any ports or services so I am hoping the SIM card will provide similar functionality.
Looking to use an Amaysim data only SIM plan (uses Optus 4G).
You would have to find another Amaysim customer in your local area who is also running his own mail server to be 100% sure that things will work. Their sales folks *might* give straight answers by phone or chat online.
With the exception of SMTP traffic on port 25 I would not expect them to be blocking anything -- most early adopters are tech folks who would need to be able to make connections on random ports.
SMTP traffic on port 25 is a different issue, and may be blocked: outbound (because they don't want customers sending SPAM from their IP block), inbound (because they don't want customers hosting their own mail servers), or both.
If SMTP traffic is blocked would have to reconfigure your SME to relay outgoing mail through a smarthost somewhere to handle outgoing email, and you might need to configure an external mail service to collect your email then reconfigure the SME to use 'fetchmail' go get your mail from the external server. Amaysim sales/support should be able to tell you if you can host your own mail server behind their service.
If it turns out that Amaysim is blocking ports that you need for your SME server, you could possibly setup Amaysim at the office for your Windows users, but move the SME server physically to another location where the internet still works. (This, of course, introduces a host of other technical issues -- but your email would work...).
Good luck.