Telecom VDSL - home network design (wireless, gigabit switches, etc.)

I have a DSL 2870B wireless gigabit ADSL modem/router (ftp://files.dlink.com.au/products/DSL-2870B/REV_A/Manuals/DSL-2870B_A1_Manual_v2.00.pdf).

I am about to hook up to Telecom's VDSL service. They have supplied the TG589vn vdsl modem ( https://static.telecom.co.nz/digital-content/assets/documents/technicolor_tg589vn_user_guide.pdf). This does wireless and has 4 LAN ports, but only 100 Mbps.

My home network is wired cat 5e and there are 3 PCs, an XBOX and a few wireless devices I stream content to. (As an aside, there is also a Samsung TV but that and the xbox are connected to the 5 port switch as there is only one RJ45 in the TV room and I never use both simultaneously.)

I currently use the DSL 2870B for everything - it has an ADSL port and 4 x 1000Mbps LAN ports so works a charm.

Presumably best bet would be to buy an 8 port switch, but was wondering if I could:

Connect VDSL modem to one of the DSL 2870 ethernet ports.

Connect Xbox and 2xPC to the remaining 3 DSL 2870

Continue to use DSL2870 for wireless (given it has a gigabit connection to the network, whereas the new ADSL modem is only 100Mbps)

Connect remaining PC to the 100Mbps LAN port on the back of the new VDSL modem (speed not a major for this PC). If this is a deal-breaker I could buy a cheap 100Mbps switch and presumably connect the VDSL and remaining PC to this switch, and then connect this switch to the DSL 2870B?

Would this work? Presumably I'd have to play around with some settings to avoid ip conflicts? I'm assuming I can't use the ADSL port on the back of the DSL 2870B as a LAN port, as that would give me the 5th 1000Mbps port I need.

Thanks in advance.Cheers

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