Hello, I have this current set up

Building #1 has wAP LTE6 kit
DHCP server enabled 192.168.88.0/24
WAN is lte1
wlan2 is "ap bridge" mode
bridge: wlan + eth (without lte)

Building #2 has SXTsq 5 ac
WAN is wlan1
wlan1 is "station bridge" mode and connected to wAP LTE6 kit wlan2
ether1 is connected to Reolink switch (there are few cameras connected to this Reolink switch)
bridge: wlan + eth

Everything works as it should.

But building #2 has ISP Telia router, I want to use this as backup internet connection. It is not always 24/7 available which is why I don't want to make it as main internet connection.
I could connect Telia router to this Reolink switch.
Traffic from Telia router has to be routed to wAP LTE6 kit, so when LTE link is down, then wAP would use internet access via Telia router. I guess?
Problem is that there would be 2 DHCP servers (Telia router and wAP LTE6 kit), so any devices connected should not get IP address from Telia router.
Telia router DHCP server uses 192.168.1.0/24

Is it possible to set up that way?

edit: I think that's not possible, because any devices connected to Reolink switch can get IP address from Telia router (instead of wAP LTE6 kit). SXTsq 5 ac has only one ether port. I could swap devices though, wAP LTE6 kit has 2 ether ports (for example eth1 for Telia router and eth2 for Reolink switch)

I would also do it the way you suggested and create a failover situation:
Or if you want to check connectivity on lte recursively:
Note: Set add-default-route=no in the LTE settings if it's turned on: