My broadband modem is a Draytek Vigor 167, which worked out of the box. It is said to have an admin interface at 192.168.1.1.

Is it possible to access it via my Mikrotik Hex? The hex decided to create a route with DAC to 192.168.1.0 using gateway public-subnet (which is the bridge comprising ether2 and ether6 - respectively the port connected to the Draytek modem and the port connected to a switch which supports a number of computers using a block of public IP addresses). It is showing as reachable and preferred source is 192.168.1.99.

I have added an address of 192.168.1.99/24 for network 192.168.1.0 and interface public_subnet. Also, a firewall entry for the forward chain to accept packets with destination 192.168.1.1 and source list LAN.

But I don't get to access the modem admin interface. Am I doing something wrong, or missing something?

I do the same thing - different modem. First step is to make sure that the modem REALLY has an admin interface on 192.168.1.1 Take a computer and give it an address in the 192.168.1.n subnet and plug it directly into the modem LAN connection. Make sure you can access the admin interface. Once you KNOW that there really is an admin interface to be found on 192.168.1.1, then you can proceed with getting to it through your router.
In my case, my Spectrum cable modem is configured in bridge mode so my router gets a public IP as opposed to something non-public. The router admin is accessed at 192.168.100.1. The router operates as a DHCP client on the cable internet. So first is the DHCP client: Then firewall rule to allow access to the model - in my case only from IP addresses on list
If you have rules to block private address ranges from coming from the internet power, make sure to specifically allow the needed address.

I think that's all I had to do...

Thanks for the sensible advice. For once I'd RTFM and it says in BOLD that the IP address is 192.168.1.1. And I assumed that my limited skills with RouterOS would be the problem. But connecting a laptop directly didn't work. Much searching threw up lots of pages saying that Draytek modems use 192.168.1.1. Finally a page said that the Vigor 166 actually uses 192.168.2.1. It seems that the VIgor 167 does too. With the correct address it was easy to get it all working. Thanks again!

Glad my first suggestion was the right one. I have been caught too many times with something not being on the IP common knowledge said it should be... I always test if the obviousl does not work.