Well old firmware can fail to connect and can trigger an IP block.
Some Yealink firmware is too old and needs an intermediate firmware…if you can’t find it I can look at work tomorrow.
FWIW we use DHCP reservations in almost all settings, especially after that one Windows 10 feature update that lost any configured fixed IP. Except we haven't set up DHCP in our data center...several of the web servers have more than one IPv4 address and all have a range of IPv6 allocated. As...
3CX has a built in firewall test…
Port 443 is the default now. 5001 implies an older install? AFAIK the supported way to change web server ports is to reinstall.
I don’t recall if parking orbit https://www.3cx.com/docs/pbx-dial-codes/ is enabled on multitenant? I don’t think I’ve ever used it since SP is so much easier. You’ve programmed SP BLFs for them?
Neither Free nor SMB will have SIP phone support. I do not know at what point that will change for SMB which historically has allowed desk phones. See this blog post, which is talking about 4SC (Basic) Free in that paragraph, per other threads.
No, there's a test in the admin panel but it's not a concern if it's 3CX hosting.
3CX used to say router phones could handle up to 10 so either way 14 is too many. You can have multiple router phones per location, or they could all be router phones.
Does the firewall test pass on the server?
Are these set up as router phones or using an SBC?
Try the connection/call quality monitor and see what it says for problem calls.