If it's as straight forward as you've described and your old router has a WAN port you should just be able to carry on using that. The only time you can't escape using the Smart Hub 2 (SH2) is if you need Digital Voice, as that has to go through the SH2.
Edit: ...but again, you haven't explained why you feel you need another router?
Exactly, why do you need more ports on the router when you can have as many as you like on a switch(es)?
The trouble is people say things on here without really understanding what they are saying. So, if you do need two routers the implication is that you have two internet connections? Is that for load balancing or failover?
If cost is not an issue TP link do a 28 Port POE switch on Amazon for £180 which should more than cover your needs.
It would help if the exact problem was described.
Having pondered what you’ve said so far, I think you really just need another switch, not a “router”. (The “router” provided in the domestic market is a cheapo multipurpose box that does a number of jobs that would be separate boxes in industry. As routing is the most significant, it gets the name router but the routing function is not what you want here, I think).
What you need (I think) is room to connect a second switch to the SH2. It is possible to connect one switch after another in what is known as a cascade. (Price is that it pushes latency up a little bit). If so, I would do it this way.
Current setup:
Router port
1 to shed switch?
2 BT halo WiFi extender
3 Electric gates
New setup:
Router port
1 to new switch…………………….one port on that to the shed, the other ports to what you want.
2 BT halo WiFi extender
3 Electric gates